If you have any 3rd party UFO data files in your TA game folder when you install TA:M, the program will consider them "active" and they will show up on the UFO Tab and be highlighted. You can deactivate them by clicking them from this tab. If you deactivate them, the files are moved to the UFOManager folder.

It is also important to note that any UFO data files that do not have a matching .txt description file will be considered to be compatible with any active TC.
TC compatibility codes are useful to give you warnings that the UFO will not be compatible with the currently active TC.
Learn about making UFO description files in the "How do I make TA:M handle my UFOs" section.