I have several projects but I start test plans with test suites which contain sections of my projects test cases. So for instance I have a consumer project with all my test cases in it via sections and subsections. I set up a test plan and test suite under a different project (consumer android for instance for test plan and test suites broken down to parts of the consumer android) - settings, etc which I copy test cases/sections into from my overarching project related to each section. Then I run the test cases inside the plan.
When plan is completed you close that. I don't know about closing a project. I could be missing something but think it's plans that you can close (and no longer make any changes to stuff inside it (test cases, etc).
Is that what you are thinking of?