Paul named it CUSF (which is a pretty horrible name), but basically means SF without merging to core repo. You can run a vanilla core node and still enforce a soft fork by running an out-process validator.
to play the devil’s advocate: there is also the issue of people not wanting to upgrade too quickly and easily. out-process activation makes it easier to only review the new rules enforced instead of every code change. this also solves signaling for soft forks with older versions.