rollover IRA

401K -> Traditional IRA -/-> (cannot rollover again) to another 401K.

401K -> Rollover IRA  -> (can still rollover to ) another 401K.

For example, you worked for A, then quit.  You can rollover A's 401K to Tradtional IRA or Rollover IRA.

next year, you works for B.  You cannot rollever money in Tradtiional IRA back to B's 401K. 

However, you can rolloever money in Rollover IRA to B's 401K.  

But nobody wants to put IRA money back to 401K, you lose investment choice with nothing to gain.

So rollever IRA is not that useful.