There is no bright-line definition. I agree that the plan administrator has to figure it out based on all the circumstances. However, I suggest that a self-employment situation, including (or especially) a law firm, is full of opportunities to abuse, so the plan administrator should be looking for a hook for start of required distributions. In particular, any change in any perquisites should be examined. I have been through it, and it is not easy because the arrangements can be nebulous and not consciously abusive. What constitutes "retirement" of a venerable law partner is an issue for law firms beyond 401(a)(9). Remember the purpose of the law.