There are two file formats that use the .loc extension. The TopoGrafix Binary format was used in earlier versions of ExpertGPS. It has been replaced by GPX, the GPS Exchange format, which is an open standard for exchanging GPS data. Geocaching.com also generates files with the .loc extension. This format was jointly developed by TopoGrafix and Geocaching.com to allow ExpertGPS users to download geocaches to their GPS receivers.
ExpertGPS can open both types of LOC files. When you save the file, you will be asked to save in GPX format.