System.runAs() is used in Apex tests to execute a block of code in the context of a specified user so that user-dependent record-sharing behavior can be verified. It is particularly useful when the application relies on a Private OWD, role hierarchy, manual or managed shares, or classes declared with sharing. A well-designed test creates representative users, runs the relevant code under those users, and confirms that authorized users can retrieve the intended records while unauthorized users cannot. The key distinction is that runAs() is not itself a production security mechanism and does not replace explicit enforcement of CRUD or Field-Level Security. Object permissions, field permissions, and record sharing remain separate layers that the implementation must enforce appropriately. This question therefore focuses on the sharing aspect of user context rather than on field-level authorization. Study Guide reference: Access to Records - Apex sharing tests, System.runAs(), user context, record visibility, and secure testing of sharing behavior.