The departure certificate issued by Russia’s Social Fund (SFR), the Russian counterpart of an A1. It has to state that Russian legislation continues to apply to you while you work remotely from Spain, with the start and end dates of that period. A document that only confirms you are registered in the Russian system does not cover the point, and private health insurance is a separate requirement.
The certificate is issued to you in person or to a representative holding a notarial power of attorney; there is no online procedure. An offer to “get it done in Moscow” without that power of attorney is a forgery: UGE checks the certificate with the issuing office and refuses the application.
Request it first rather than last. If UGE asks for it in a requerimiento, you have 10 working days to answer and up to 5 more on request (Ley 39/2015, art. 68) — rarely enough to obtain the certificate from scratch. How those requests read and how to answer them: the UGE requerimiento.
Three Russian documents are regularly submitted in its place and none of them work: an individual account statement, an ADI-REG notice and a SNILS card. Each confirms only that you are registered in the Russian system, which is not the question UGE is asking.