From 2013 on there is a new system (Se cambiarán chapas de identificación de todos los vehículos cubanos http://www.cubadebate.cu/noticias/2013/04/27/se-cambiaran-las-matriculas-chapas-de-todos-los-vehiculos-cubanos/).

It consists of a single letter and 6 numbers (for cars at the moment) or 5 numbers (for motorcicles) to the right. Ex: P 001898

This new convention does not differentiate between provinces (as before) and there is a small subset of background colors (in the past there were like 6 background colors meaning several things like if it was a diplomat car, private one, etc)

In the current definition the first letter corresponds to:

A – Government vehicles assigned to certain persons or duties
C, D and E : Vehicles for diplomats (I think D is for diplomat personel, C for consular and E for Specialists/foreign technicians).
M: Ministry of the interior (policial forces and other law enforcement forces)
F: Army (Fuerzas Armadas)
K: Foreign particulars (foreign persons, not specifically diplomats but for foreign residents in Cuba). K is not a common letter in Spanish and it is mostly used for foreign words, maybe this is the reason for the K for foreigners.
T: Tourism
All the other car types will be assigned with the rest of the letters. However I have noticed that the P is for cuban particulars (cuban private owners of the cars) and B is being assigned to state companies. The state owned vehicles will have a Blue background where the word “Cuba” goes. I guess as soon as they reach P 999999 probably a new letter will be issued for particulars. As well as probably when they reach B 999999 a new letter will be used for state companies.
For some unexplained reason (at least to me) I, O, Q, W, S and Z wont be used. I guess it is because I gets confused for a 1, and O and Q tend to be confused with 0.