This format can...
read and write waypoints
read and write tracks
read and write routes
Garmin documents only PCX5, an older format limited to the lame NMEA six-character waypoint names that's treated as a second-class citizien in current versions of MapSource. In Mapsource, use file->import to read these files. If you name the files *.wpt, Mapsource will find them more easily.
In general, you should prefer the "mapsource" file format to this one.
This format has been extended to handle many - but not all - files from GPS Utility. If you encounter something that GPSBabel does not handle well, use the free version of GPSUtil to read it and save as something more common.