query filter for Test Plans, Test Cases or Test Scripts

You can create a category for test plan, test  case etc with project name as its values. then when creating testplan, test cases or test script, if you fill the category with the right project name, you can then filter the artifacts by the project name category. Some filter info can be found in help:
https://pic.dhe.ibm.com/infocenter/clmhelp/v4r0m3/index.jsp?re=1&topic=/com.ibm.rational.test.qm.doc/topics/t_filtering_by_categories.html&scope=null

If you want to know more about category, you can search for category in the above help site.
Hopefully this helps.