RFC 1247 mandated that routes associated with Area A are never summarized back into Area A. However, this memo further reduces the number of summary links originated by refusing to summarize into Area A those routes having next hops belonging to Area A. This is an optimization over RFC 1247 behavior when virtual links are present. For example, in the area configuration of Figure 6, Router RT11 need only originate a single summary link having the (collapsed) destination N9-N11,H1 into its connected transit area Area 2, since all of its other eligible routes have next hops belonging to Area 2 (and as such only need be advertised by other area border routers; in this case, Routers RT10 and RT7). This is the logical equivalent of a Distance Vector protocol's split horizon logic.
This change appears in Section 12.4.3.