Yes, "use" means to check-in, board, and fly.
If you check-in and don't fly, it will still be unused, and
your subsequent flights can be cancelled with no recourse.

If you're late for the first segment of your airpass
because your transatlantic is late, your flights might
be cancelled, but they should reinstate everything
since it's a valid delay. If this does happen, make
sure you verify with them that your subsequent
flights haven't been cancelled. You won't be accorded the
same courtesy if you check-in and don't fly.