Explanation
The annotation @Environment.systemField: #LANGUAGE is used to assign the ABAP system field sy-langu to an input parameter of a CDS view or a CDS table function. This enables the implicit parameter passing in Open SQL, which means that the value of sy-langu will be automatically passed to the CDS view without explicitly specifying it in the WHERE clause. This also applies to the CDS views that use the annotated CDS view as a data source, which means that the value of sy-langu will be propagated to the nested CDS views (view on view)12. For example:
The following code snippet defines a CDS view ZI_FLIGHT_TEXTS with an input parameter p_langu that is annotated with @Environment.systemField: #LANGUAGE:
define view ZI_FLIGHT_TEXTS with parameters p_langu : syst_langu @<Environment.systemField:
#LANGUAGE as select from sflight left outer join scarr on sflight.carrid = scarr.carrid left outer join stext on scarr.carrid = stext.carrid { sflight.carrid, sflight.connid, sflight.fldate, scarr.carrname, stext.text as carrtext } where stext.langu = :p_langu The following code snippet shows how to use the CDS view ZI_FLIGHT_TEXTS in ABAP without specifying the value of p_langu in the WHERE clause. The value of sy-langu will be automatically passed to the CDS view:
SELECT carrid, connid, fldate, carrname, carrtext FROM zi_flight_texts INTO TABLE @DATA(lt_flights).
The following code snippet shows how to use the CDS view ZI_FLIGHT_TEXTS in another CDS view ZI_FLIGHT_REPORT. The value of sy-langu will be automatically passed to the nested CDS view ZI_FLIGHT_TEXTS:
define view ZI_FLIGHT_REPORT with parameters p_langu : syst_langu @<Environment.systemField:
#LANGUAGE as select from zi_flight_texts(p_langu) { carrid, connid, fldate, carrname, carrtext, count(*) as flight_count } group by carrid, connid, fldate, carrname, carrtext The annotation @Environment.systemField: #LANGUAGE does not prevent the possibility of overriding the default value with a value of your own. You can still specify a different value for the input parameter p_langu in the WHERE clause, either in ABAP or in another CDS view. This will override the value of sy-langu and pass the specified value to the CDS view12. For example:
The following code snippet shows how to use the CDS view ZI_FLIGHT_TEXTS in ABAP with a specified value of p_langu in the WHERE clause. The value 'E' will be passed to the CDS view instead of the value of sy-langu:
SELECT carrid, connid, fldate, carrname, carrtext FROM zi_flight_texts WHERE p_langu = 'E' INTO TABLE @DATA(lt_flights).
The following code snippet shows how to use the CDS view ZI_FLIGHT_TEXTS in another CDS view ZI_FLIGHT_REPORT with a specified value of p_langu in the WHERE clause. The value 'E' will be passed to the nested CDS view ZI_FLIGHT_TEXTS instead of the value of sy-langu:
define view ZI_FLIGHT_REPORT with parameters p_langu : syst_langu @<Environment.systemField:
#LANGUAGE as select from zi_flight_texts(p_langu) { carrid, connid, fldate, carrname, carrtext, count(*) as flight_count } where p_langu = 'E' group by carrid, connid, fldate, carrname, carrtext References: 1: ABAP CDS - parameter_annot - ABAP Keyword Documentation - SAP Online Help 2: ABAP CDS - session_variable - ABAP Keyword Documentation - SAP Online Help

Explanation
When processing an internal table with the statement LOOP AT itab... ENDLOOP, the system variable that contains the current row number is sy-tabix. The sy-tabix variable is a predefined field of the system structure sy that holds the index or the row number of the current line in an internal table loop. The sy-tabix variable is initialized with the value 1 for the first loop pass and is incremented by 1 for each subsequent loop pass. The sy-tabix variable can be used to access or modify the current line of the internal table using the index access12.
References: 1: LOOP AT itab - ABAP Keyword Documentation - SAP Online Help 2: System Fields - ABAP Keyword Documentation - SAP Online Help

Explanation
The object that can be used to set a field to read-only in all applications of the RESTful Application Programming model (RAP) is the behaviour definition. The behaviour definition is a CDS artefact that defines the business logic and the UI behaviour of a business object. A business object is a CDS entity that represents a business entity or concept, such as a customer, an order, or a product. The behaviour definition can specify the properties of the fields of a business object, such as whether they are mandatory, read-only, or transient. These properties are valid for all applications that use the business object, such as transactional, analytical, or draft-enabled apps12. For example:
The following code snippet defines a behaviour definition for a business object ZI_PB_APPLICATION.
It sets the field APPLICATION to read-only for all applications that use this business object:
define behavior for ZI_PB_APPLICATION { field ( read only ) APPLICATION; ... } You cannot do any of the following:
A). Service definition: A service definition is a CDS artefact that defines the interface and the binding of a service. A service is a CDS entity that exposes the data and the functionality of one or more business objects as OData, InA, or SQL services. A service definition can specify the properties of the fields of a service, such as whether they are filterable, sortable, or aggregatable. However, these properties are only valid for the specific service that uses the business object, not for all applications that use the business object12.
C). Projection view: A projection view is a CDS artefact that defines a view on one or more data sources, such as tables, views, or associations. A projection view can select, rename, or aggregate the fields of the data sources, but it cannot change the properties of the fields, such as whether they are read-only or not. The properties of the fields are inherited from the data sources or the behaviour definitions of the business objects12.
D). Metadata extension: A metadata extension is a CDS artefact that defines additional annotations for a CDS entity, such as a business object, a service, or a projection view. A metadata extension can specify the properties of the fields of a CDS entity for UI or analytical purposes, such as whether they are visible, editable, or hidden. However, these properties are only valid for the specific UI or analytical application that uses the metadata extension, not for all applications that use the CDS entity12.
References: 1: ABAP CDS - Data Definitions - ABAP Keyword Documentation - SAP Online Help 2: ABAP CDS - Behavior Definitions - ABAP Keyword Documentation - SAP Online Help

Explanation
The nested join is evaluated from the top to the bottom in the order of the ON conditions. This means that the join expression is formed by assigning each ON condition to the directly preceding JOIN from left to right.
The join expression can be parenthesized implicitly or explicitly to show the order of evaluation. In this case, the implicit parentheses are as follows:
SELECT * FROM (a INNER JOIN (b INNER JOIN c ON b~c = c~c) ON a~b = b~b) This means that the first join expression is b INNER JOIN c ON b~c = c~c, which joins the columns of tables b and c based on the condition that b~c equals c~c. The second join expression is a INNER JOIN (b INNER JOIN c ON b~c = c~c) ON a~b = b~b, which joins the columns of table a and the result of the first join expression based on the condition that a~b equals b~b. The final result set contains all combinations of rows from tables a, b, and c that satisfy both join conditions.
References: 1: SELECT, FROM JOIN - ABAP Keyword Documentation - SAP Online Help

Explanation
The data declarations that will always work without truncation or rounding for the assignment gv_target = gv_source are B and C.
This is because the target data type string is a variable-length character type that can hold any character string, including those of data types c (fixed-length character) and d (date). The assignment of a character or date value to a string variable will not cause any loss of information or precision, as the string variable will adjust its length to match the source value12.
You cannot do any of the following:
A). DATA gv_source TYPE string, to DATA gv_target TYPE c.: This data declaration may cause truncation for the assignment gv_target = gv_source. This is because the target data type c is a fixed-length character type that has a predefined length. If the source value of type string is longer than the target length of type c, the source value will be truncated on the right to fit the target length12.
D). DATA gv_source TYPE p LENGTH 8 DECIMALS 3. to DATA gv_target TYPE p LENGTH 16 DECIMALS 2.: This data declaration may cause rounding for the assignment gv_target = gv_source.
This is because the target data type p is a packed decimal type that has a predefined length and number of decimal places. If the source value of type p has more decimal places than the target type p, the source value will be rounded to the target number of decimal places12.
References: 1: ABAP Data Types - ABAP Keyword Documentation - SAP Online Help 2: ABAP Assignment Rules - ABAP Keyword Documentation - SAP Online Help