In ABAP SQL, an alias is a temporary name that can be assigned to a field or a database table in a query. An alias can be used to make the query more readable, to avoid name conflicts, or to access fields or tables with long names. An alias is created with the AS keyword and is only valid for the duration of the query1.
The following are examples of how to assign an alias to a field or a database table in ABAP SQL:
* B. field (from field list): A field is a column of a table or a view that contains data of a certain type. A field can be assigned an alias in the field list of a SELECT statement, which specifies the fields that are selected from the data source. For example, the following query assigns the alias name to the field carrname of the table scarr:
SELECT carrid, carrname AS name FROM scarr.
The alias name can be used instead of carrname in other clauses of the query, such as WHERE, GROUP BY, ORDER BY, and so on2.
* C. database table: A database table is a collection of data that is organized in rows and columns. A database table can be assigned an alias in the FROM clause of a SELECT statement, which specifies the data source that is selected from. For example, the following query assigns the alias c to the table scarr:
SELECT c.carrid, c.carrname FROM scarr AS c.
The alias c can be used instead of scarr in other clauses of the query, such as WHERE, JOIN, GROUP BY, ORDER BY, and so on3.
The following are not valid for assigning an alias in ABAP SQL:
* A. order criterion (from order by clause): An order criterion is a field or an expression that is used to sort the result set of a query in ascending or descending order. An order criterion cannot be assigned an alias in the ORDER BY clause of a SELECT statement, because the alias is not visible in this clause. The alias can only be used in the clauses that follow the clause where it is defined1.
* D. group criterion (from group by clause): A group criterion is a field or an expression that is used to group the result set of a query into subsets that share the same values. A group criterion cannot be assigned an alias in the GROUP BY clause of a SELECT statement, because the alias is not visible in this clause. The alias can only be used in the clauses that follow the clause where it is defined1.
References: 1: ALIASES - ABAP Keyword Documentation 2: SELECT List - ABAP Keyword Documentation 3: FROM Clause - ABAP Keyword Documentation

The annotation that can be used to provide a short description of the data definition for developers that will be attached to the database view is the @EndUserText.label annotation. This annotation is used to specify a text label for the data definition that can be displayed in the development tools or in the documentation. The annotation can be inserted on line #27 in the code snippet provided in the question12. For example:
* The following code snippet uses the @EndUserText.label annotation to provide a short description of the data definition for the CDS view ZCDS_VIEW:
@AbapCatalog.sqlViewName: 'ZCDS_VIEW' @AbapCatalog.compiler.compareFilter: true
@AbapCatalog.preserveKey: true @AccessControl.authorizationCheck: #CHECK @EndUserText.label:
'CDS view for flight data' "short description for developers define view ZCDS_VIEW as select from sflight { key carrid, key connid, key fldate, seatsmax, seatsocc } You cannot do any of the following:
* @UI.headerInfo.description.label: This annotation is used to specify a text label for the description field of the header information of a UI element. This annotation is not relevant for the data definition of a database view12.
* @UI.badge.title.label: This annotation is used to specify a text label for the title field of a badge UI element. This annotation is not relevant for the data definition of a database view12.
* @EndUserText.quickInfo: This annotation is used to specify a quick information text for the data definition that can be displayed as a tooltip in the development tools or in the documentation. This annotation is not the same as a short description or a label for the data definition12.
References: 1: ABAP CDS - SAP Annotations - ABAP Keyword Documentation - SAP Online Help 2: ABAP CDS - Data Definitions - ABAP Keyword Documentation - SAP Online Help

The following are the explanations for each statement:
* C: This statement is valid. Class CL1 uses the interface. This is because class CL1 implements the interface ifl using the INTERFACES statement in the public section of the class definition. The INTERFACES statement makes the class compatible with the interface and inherits all the components of the interface. The class can then use the interface components, such as the method ml, by using the interface component selector ~, such as ifl~ml12
* E: This statement is valid. Class CL1 implements the interface. This is because class CL1 implements the interface ifl using the INTERFACES statement in the public section of the class definition. The INTERFACES statement makes the class compatible with the interface and inherits all the components of the interface. The class must then provide an implementation for the interface method ml in the implementation part of the class, unless the method is declared as optional or abstract12
* D: This statement is valid. In class CL2, the interface method is named ifl~ml. This is because class CL2 has a data member named m0_ifl of type REF TO ifl, which is a reference to the interface ifl. The interface ifl defines a method ml, which can be called using the reference variable m0_ifl. The interface method ml has the name ifl~ml in the class, where ifl is the name of the interface and the character ~ is the interface component selector12 The other statements are not valid, as they have syntax errors or logical errors. These statements are:
* A: This statement is not valid. In class CL1, the interface method is named ifl~ml, not if-ml. This is
* because class CL1 implements the interface ifl using the INTERFACES statement in the public section of the class definition. The interface ifl defines a method ml, which can be called using the class name or a reference to the class. The interface method ml has the name ifl~ml in the class, where ifl is the name of the interface and the character ~ is the interface component selector. Using the character - instead of the character ~ will cause a syntax error12
* B: This statement is not valid. Class CL2 does not use the interface, but only has a reference to the interface. This is because class CL2 has a data member named m0_ifl of type REF TO ifl, which is a reference to the interface ifl. The interface ifl defines a method ml, which can be called using the reference variable m0_ifl. However, class CL2 does not implement the interface ifl, nor does it inherit the interface components. Therefore, class CL2 does not use the interface, but only references the interface12 References: INTERFACES - ABAP Keyword Documentation, CLASS - ABAP Keyword Documentation