Once a unit reaches the table edge it has one more chance to regroup, if it fails it is removed and may not return. Note that there is no need for individual models in the unit to move 'off' the table, once any reach the edge it is last chance time! Finish the move for all the models in the unit, stopping any who would finish off the table edge, and then attempt to regroup, as long as none of the usual restrictions apply. If the unit regroups it stops at the table edge. If it fails it is removed.
Last Chance
Source: Core Rulebook 3E, p. 73
A unit that reaches the table edge while falling back gets one final regroup attempt before being removed from the game.