I have created First Project and Add 10 test case i.e. C1 to C10. Now I have created another project and test case ID start from C11. Is it possible to give test case ID C1 again for new Project?
No I don’t think so (i could be wrong).
In the testrail database each test case id is unique, therefore allowing you to use references from one test case to another i.e. [C2]. when viewing a script you would be able to click the reference link which takes you automatically to the other test case as in the example shown.
Hope this helps
Another reason for the unique case numbers is because test cases can be moved from one project to another. If a case is moved between projects it doesn’t need to be renumbered.
Yes, IDs in TestRail are unique and it would be problematic if the IDs would start at C1 again for a new project. With the unique numbering, it’s very easy to refer to a test case using its ID and you can uniquely identify a test case this way. If the same ID would be issued multiple times, TestRail wouldn’t know which test case you mean if you link to a test case, for example.