There appears to be a comment flag on my earlier post suggesting that what I wrote was confusing. Perhaps that is because I was trying to say that solo plans are, by definition, confusing because the word "solo" does not appear to have a uniform definition across the industry.
From my view, the word "solo" implies only that the plan was optimized (whatever that means) in some way for small businesses (whatever that means). Such optimization is not required by law but is discretionary for the vendor. For example, one vendor has a DC plan available for every employer, and also has an alternative DC plan available for any company that wishes to use the alternate DC plan, and the latter product is something that vendor calls a "solo" plan. The only reason that vendor is offering such a plan is because there is a market that generally coincides with the employers that historically would have used a Keogh plan. Rather than forcing such employers use the equivalent of a Keogh plan, they might have a plan that is much more flexible that a Keogh plan. Each vendor will take its own course, which is why I cannot answer questions about solo plans (as if all solo plans are the same) unless I can examine the document (since not all solo plans are the same). The word "solo" in and by itself offers no clue as to the nature or the extent of how a plan product has been optimized for the targeted market, but tells me only that such optimization has occurred. It therefore would not surprise me if I saw a solo product that didn't cover common law employees or that prohibited MEPs, but I would not make such assumptions.
For example, I believe that you will not find any reference to solo plans in any regulation or IRS Revenue Procedure (not even the Revenue Procedure for Cycle 3 and which governed the approval of Cycle 3 solo plan products). I am relying on Window's file search function in making this statement (I have a folder that contains 90% of the guidance issued on retirement plans over the past decade). In contrast, when I do a general search for "solo plan" on the internet, I find many listings on the first page, each presenting a different definition, such as "an account for the self-employed," or "a single-participant plan," etc. My favorite one states: "A solo 401(k) is a traditional 401(k) plan, except that it covers only one employee -- the sole proprietor of a business -- and, at most, a spouse."
Notwithstanding such definitions, the solo plan with which I am the most familiar covers every employee other than certain statutorily excluded employees (e.g., nonresident aliens with no US income), with no option to exclude any more. If I used only the internet for my answers, then the solo plan I am using shouldn't be called a solo plan. The IRS didn't mind, though, because they (apparently) don't have a definition of solo when reviewing solo plans.
Consequently, when I see an inquiry that poses an inquiry regarding solo plans as if all solo plans are alike, I am usually quick to indicate to that person that I am unable to generalize anything about the nature of the solo plan without knowing a great deal more about THAT particular solo plan. For example, knowing that the plan is a solo plan does not inform me of the nature or scope of that vendor's optimization for the targeted market. The fact that it is a solo plan does not tell me what consequences will arise if another employer adopts the plan or if any adopting employer hires an employee. Even if that vendor's software does not automate any additional employer participation agreements, it is conceivable that the BPD not only allows additional adopting employers, but also allows MEPs. That means I must scour each specific "solo" document to answer questions about the nature and degree of its being "solo," such as its restrictions on additional participating employers and on which employees may be or must be covered (or cannot be covered).