Hi WJF,
I can only tell you of my experience.
Both Cla@ve and Electronic Certificates (ACCV, FNMT) can be used to access a variety of Spanish authorities' websites and conduct business online which you would otherwise have to do in person. I tried to apply for a Cl@ve and could only get so far before I kept getting an error message. At the time, advice from another member was to not bother with a Cl@ve - it's clunky to use - and an Electronic Certificate is much more user-friendly. If I remember correctly, Cl@ve originated in the Agencia Tributaria tax office. The ACCV is issued by the Certification Authority of the Valencian Community and the FNMT is issued by Spain's Royal Mint (Ministry of Finance).
I recommend the ACCV certificate because, initially, you have to present yourself in person to prove your identity and going to your Town Hall is probably the easiest place to do that. Once you've obtained the Electronic Certificate and installed it, you'll also need to download AutoFirma software from the internet (it's free), in order to digitally sign/authorise your online transactions, e.g. updating your address with any government department and agency, applying for a Padron certificate, checking your driving licence records, submitting your tax returns, etc.
Electronic certificates can be used on multiple devices - I have mine and hubby's loaded on our desktop and laptop, and we each have our own on our smartphones. I've had my ACCV certificate for about 6 months - at that time, I was able to load it and use it with Firefox, but it didn't work with Google Chrome, although that may have changed by now, as the certificate authorities extend their scope.
Because of Covid-19 restrictions, you'll probably need to make an appointment with your Town Hall to obtain your unique 25-digit code, with which you should also receive instructions about how to install the certificate.
Kind regards,
Kim