This is more of an alert than a question. I don't know if this is Massachusetts specific.
Your self-employed and gig economy clients may be asking for your help responding to the request for income verification. Read the Fine Print.
Hidden near the end of the emailed request, in the What Happens While My Documents are Being Reviewed section, in parenthesis. they explain the Base Year is the year before the worker first applied for unemployment.
So, if they applied and received 2020 Unemployment, they want 2019 information. It makes sense, but is not obvious when the list of documents to confirm income comes right before the add and upload screen.
Once submitted, you cannot upload additional documents to correct your error without calling the agency.