MI13 Reconnaissance MI14 and MI15 German specialists, Mi16 royal secret service MI17 Military Intelligence "Head Office". MI19 Po W debriefing, A full list of Miliary Intelligence (MI) Departments during world war 2 can be found on pages 147 and 148 of "Codebreaker in the Far East" by Alan Stripp, published in 1989 by Oxford University Press.
This goes numerically up to MI19 plus MIL, MIR and MIX.
MI4 Aerial Reconnaisance MI5 domestic intelligence MI6 foreign intelligence MI7 Propaganda MI8 Military Communication Interception, MI9 Undercover operations, /POW escape MI10 Weapons analysis MI11 Field security police MI12 ???
I believe the difference is like the FBI and CIA in the USA - one is for domestic intelligence and one is for international intelligence. I also think that some of the other numbers may have been active in intelligence and of the like during the world wars.