I got the "A" service due indication in my 2007 RDX base. No number.
What is service A?
I have almost 44K.

 

It tells you right in your owner's manual. I'm not sure exactly what it is at the moment, as I don't have my manual in front of me. But, all of the service codes are explained in the owner's manual. Take a look!

 

The manual only has explanations by number, A1, A2, etc. But not without the numbers. I hope it's not all of the above.
However I am close to that comprehensive service point, which requires flush/replace all fluids, etc...

 

A is replace oil (does not include filter) from Acura. Do it your self and replace the filter as well!