Until now, teams had the option of the traditional onside kick & scrum, or converting a 15 yard 1st down (in one play.) The UFL will now only allow onside-by-conversion, but shorten the line of gain from 15 yards to 12, from your own 28 yard line.
https://www.nbcsports.com/nfl/profootballtalk/rumor-mill/news/ufl-ditches-onside-kicks-for-fourth-and-12-from-the-28