* The @testSetup annotation is a way to create test data for a test class. It allows you to define a method that runs once before any test methods in the class and inserts common test data into the test context12.
* The @testSetup annotation is not supported when the GisTest(SeeAllData=True) annotation is used.
The GisTest(SeeAllData=True) annotation allows a test class or method to access all data in the organization, including pre-existing data that the test didn't create. If this annotation is used, the test setup method is skipped and no test data is inserted12.
* Test data is inserted once for all test methods in a class when the @testSetup annotation is used. This means that the test data is shared across all test methods and only counts once towards the governor limits. This can improve the performance and efficiency of the test class12.
* A method defined with the @testSetup annotation does not execute once for each test method in the test class and does not count towards system limits. It only executes once before any test methods and only counts once towards the governor limits. However, any DML operations or queries performed in the individual test methods do count towards the system limits12.
References:
* 1: Using Test Setup Methods
* 2: Cert Prep: Platform Developer I: Testing and Debugging