Aa stands for Avisar a (notify) and if you programme the name and contact details of the person you want informed in case you are involved in a serious (traffic) accident in your mobile phone, the personnel from the emergency services can access this information, even when you are not in a position to inform them about it.
Using Aa as the entry in your mobile phone will make sure the relevant details will come up first in your list of contacts.
So, as well as jotting down the next of kin information in your passport, please take a minute to programme your mobile phone, to help the emergency services.
Message by the Red Cross and the Spanish Interior Ministry.