A Test Case is an executable test that consists of an immutable part extracted from the initial Job or Route, along with other editable components that form the skeleton of the Test Case.
A Test Instance is a set of data that allows you to run the Test Case with different parameters that you define (input, reference files, etc.).
It is recommended to create and use a context adapted to your environment (a Test context to execute test Jobs with the metadata of this environment, and a Production context to execute Jobs in the Production environment).
When the feature is designed and tested, it is recommended to use Talend Repository Manager to pass items to the QA environment. See Continuous Integration: Deploying to QA and Production environments for more information.