Relative of mine has been a writer for years and has used the business code for writing on both the Sched C and the
5500-EZ for the relative's solo 401k.
But in 2018 the relative started another, unrelated line of business (also as a sole prop) and so on the income tax return filed two Sched Cs, one with a business code for a writer and one with the code for the new business.
When the 5500-EZ was filed, the relative kept using the business code that had been in use (the one for writing) as 5500-EZ only has space for one code.
But what should be done? Is it arbitrary which code to use? Should you pick the one for the line of business that had the most income? Are you required to have a separate solo 401k plan for each business?