This format can...
read and write waypoints
This format is derived from the xcsv format, so it has all of the same options as that format.
This format is used by GPSBabel itself as the input to the arc and polygon filters. See those filters for more information.
The arc format reads two numeric fields, a latitude and a longitude, in any format recognized as human readable and writes as simple degrees decimal. It really is intended for GPSBabel's own internal use more than general use, though it turns out to be a convenient way of expressing simple polylines and polygons.