Starcraft seat belts

I've just bought a 1995 AWD Starcraft Astro. It is fitted with 3 point lap/diagonal seatbelt on both passenger and drivers seats as I would expect but it also has a belt with no clasp, coming out of the back of the seats, anchored on the door pillar. Can someone explain what this extra belt is supposed to do because I can't see any purpose in it. I'm sure there must be a logical reason for it.

Welcome to the forum!

The third belt is most likely for attaching to a child's car seat.