The answer seems very obvious to me, to the point that this seems like a stupid question, but sometimes I question my sanity so here goes.

Corporation A and Corporaion B form a controlled group. Corporation A sponsors a 401(k) plan, and Corporation B signs on as a participating employer. Plan # is 001.

Now Corporation B wants to set up a new plan, for reasons which have not been presented to me. So say they set up a PS plan - to which Corporation A will sign on as a participating employer. Should the plan # be 001, or 002?

I'd say 001, because this is the first plan that B will formally "sponsor" under their own id#.

Any votes for 002?

I vote for #001 (due to the EIN of Co B being different than the EIN of Co A), and you yet appear to be sane.

001. The plan identifier is the sponsor's id # plus the plan number, and I think the most important thing is that it be unique...I don't know that you must start with 001, but I would.