If true, this allows the router to leak L2 routes into L1. Enabling it lets a level1L2 border router inject backbone route information down into its level 1 area, which is what allows level-1-only routers in that area to make optimal exit choices toward multiple backbone attachment points instead of always defaulting toward the nearest L1L2 router. For example, in an area with two L1L2 border routers each learning different backbone prefixes, enabling this on both lets level-1 routers pick the closer exit point for a given destination instead of both always defaulting toward whichever L1L2 router happens to be topologically closest.