What means 2HnD in the name like this RB751G-2HnD
RB = RouterBoard
751 = Model
G = Gigabit ( i think )
2HnD = ??? ( something about WIFI ??? )
2 - 2.4ghz
H- high power
n- 802.11n
D- dual chain
Thank you!
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
L: low cost
HP: high power (more then H)
A: more RAM (usualy with Level4)
R: build in wireless