Test suite here means TOP LEVEL TEST SUITE.
We consider that Test project tree has several branches or subtree, that has as a TOP LEVEL TEST SUITE as starting point.
When you set a Test suite (remember is a TOP LEVEL TEST SUITE), search will be done only on test case that belong to this BRANCH OR SUBTREE.
20121110
If you have a Test Project like this
TPJ-01 => TSUITE-01 => TC1,TC2,TC3,TC4
=> TSUITE-02 => TC10,TC11
If you create s TEST PLAN using ONLY test cases from TSUITE-01, when you open this filter YOU ARE GOING TO SEE also TSUITE-02,
because domain is creating using TEST PROJECT CONTAINS, not TEST PLAN CONTAINS.
(hmm this may be is a BUG).