I wish the entire listing of letters was supplied.
Here is what I have so far. I compiled it be searching and guessing...
Hope it can help. If somebody from MikroTik could validate and complete, it would be greatly appreaciated. Many thanks.
i : power Injector
G : Gigabit ethernet ports
2 : 2,4GHz wifi
5 : 5GHz wifi
H : High power
52 : Both 2,4GHz and 5GHz
n : 802.11n
D : Dual chain wifi
U : USB port
RB : RouterBOARD
R : Radio interface
S : SFP port
IN : Indoor case
RM : Rackmount case
P : PoE output