The fillnull command in Splunk is used to handle missing data within search results. It plays a crucial role in data normalization and preparation, especially before performing statistical analyses or visualizations.
A: Replace empty values with a specified value: This is the correct answer. The fillnull command is specifically designed to replace null values (empty values) with a specified default value. This is particularly useful in ensuring consistency within your data, especially when performing operations that require numerical values or when you want to distinguish between genuinely missing data and zeroes, for instance.
* Example Usage: ... | fillnull value=0 This command would replace all null values in the search results with 0.

Field aliases are applied based on host, source, or sourcetype.
Extract: "Create field aliases by specifying the host, source, or sourcetype that the alias applies to." Thus, the correct answer is Sourcetypes.

The explanation is as follows:
The transaction command is used to find transactions based on events that meet various constraints12.
Transactions are made up of the raw text (the _raw field) of each member, the time and date fields of the earliest member, as well as the union of all other fields of each member1.
The startswith option is used to define the first event in a transaction by specifying a search term or an expression that matches the event13.
For example, | transaction clientip JSESSIONID startswith="view" will create transactions based on the clientip and JSESSIONID fields, and the first event in each transaction will contain the term "view" in the
_raw field2.

In this case, the outer join is applied, which means that all rows from the outer (left) table will be included, even if there are no matching rows in the inner (right) table. The result will include all five rows from the outer table, with the matched data from the inner table where employeeNumber matches. Rows without matching employeeNumber values will have null values for the fields from the inner table.
References:
Splunk Documentation - Join Command