Comprehensive and Detailed Explanation From Exact Extract:

In the ABAP RESTful Application Programming Model (RAP), validations are used to ensure that business rules and constraints are fulfilled, including the uniqueness of semantic keys.

A semantic key represents a natural identifier (e.g., employee number, product ID) and not a technical surrogate key. Validations can be:

Field-level validations – used to validate input for single fields.

Entity-level validations – used to validate logical conditions like uniqueness of a key combination.

The uniqueness check is typically enforced using a custom validation implementation in the behavior pool.

Action (Option A) is used for operations triggered by the user or system but not for enforcing uniqueness.

Determination (Option C) is used for automatically computing or adjusting field values, not for enforcing uniqueness.