When you are trying to add test cases to a Test Plan, on left pane you are accessing to Test Specification (Test project),
and all filters are applied to attributes of Test Case Specification.
You can find some a behaviour you can consider weird, when you use filter on TEST CASE VERSION attributes like, importance, execution type, Custom Fields.
Filtering is applied Latest (higher version number) active version, but on right pane when you already have a TEST CASE VERSION LINKED
that can have attributes DIFFERENT to the latest version on spec.
See document in:
https://docs.google.com/document/d/13kW ... sp=sharing
for an example