From SERVER-24538 that added IntermediateInitialSync, it sounds like that is just testing collection cloning and index building but not oplog applying phase. And SERVER-25068 (which added BackgroundInitialSync) sounds like the background hook is doing all of initial sync. It also seems like BackgroundInitialSync continuously restarts the initial syncing node, which gives us more aggressive coverage of stepdown/shutdown/recovery. IntermediateInitialSync looks like it just does one initial sync randomly during the test suite. From searching on jira, I think most of the bugs it caught were during the first year or so when both hooks were created so both suites had more BFs at that time. But it hasn't really caught anything recently