What does "Ag. Tribut." on a Correos Aviso De Llegada Mean?

Undoubtedly the Agencia Tributaria - the taxman!

If you have a digital certificate you can look for communications sent to you on the website. I would suggest you get on to them very urgently, as they are quick to levy fines for not responding within a short time.

We were in the UK when we got a message that there was a notice for us which turned out to need us to attend the tax office in Cartagena (wanted proof that my OH's pension was a government one and thus had been declared correctly - for 2015!).

It really is worth getting a digital certificate or using Clave to sign up for email notifications of any communications, especially from the Agencia Tributaria and DGT. Especially if you are often away from your postbox for any length of time. I was able to get an appointment with the taxman for after we returned to Spain, avoiding a possible €1000 fine for non-response.

Can you advise how to get a digital signature please?

Getting a digital certificate is probably best done using a gestor, as it involves appointments where you prove your identity, but if you go on to the tax site <https://www.agenciatributaria.es/AEAT.internet/en_gb/Inicio.shtml> there is a box on the left with a video explaining how to use a simpler form of access "Clave" ("key" in English. You can change the language of the website (most pages anyway) to English as soon as you enter - click top right.

Thank you