Ah, okay, that is a "standard" pointmass definition I am using in the addon to allow anyone to easily add a payload point to their aircraft and have it work in the addon. I originally copied that structure from who knows where, maybe the c172p. I assumed it to be a standard property in any pointmass definition and didn't give it a second thought. I have no idea what it is for or if it is needed or used by jsbsim. I searched the jsbsim pdf for it and it wasn't in there.
If it isn't recognised by anyone and not commonly used in other aircraft, i would suspect it might be a dead property that is no longer used? Otherwise maybe something internal to jsbsim?