A prefix is for the Title of the thread. In this case, for this forum, it would be the brand of equipment. There is a drop down just to the left of where you type in your thread title, when you make a new thread.
If my memory is correct, go into the setup for one of your satellites and arrow down to the motor type, when you have USALS highlighted hit the OK/ENTER button on the remote. This should bring up the screen to enter your Lat and Long.