Pretty sure that it relates to the vertical rating of 500 lbs. Typical recommended way to tow a trailer is to balance it out by putting 10% of the gross weight on the hitch...so if you had a 7500 lb GVW you would normally want 750 lbs on the tongue.
2009 Holiday Rambler Navigator
Cummins ISX-650/1950-tq, Allison 4000 Sold...now shopping
2007 Lexus LX470