Testable and non Testable requirements are defined based on the Testing Scope.
For ex: Consider the Application "X" which have A,B,C modules which makes the Full application "X". All the requirements are mentioned in the Design documents.
When Developers start working on the module A and the build A is availble for testing, then Module A is Testable and we can't test module B or C, so based on the development plan QA Analyst will prepare Test Strategy document and will give clear details on what requirements need to be tested and what should not.
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