While it can get hectic to figure out all the bus systems in Hong Kong, it turns out that there is some logic to some of them. The numbering scheme actually corresponds to general destinations or where the bus passes. Some things I learned today:

M### - implies that the bus will be headed to the HK MTR Station or Airport Express.
###M - implies that the bus will pass a major MTR Station or drop off really close to an MTR station

9## - signifies the bus will take the west tunnel to Kowloon
6##- signifies the bus will take the east tunnel to Kowloon
1##- signifies the bus will take the middle tunnel to Kowloon in the area of TST or Hung Hum.

The funny thing is that the guide books never mentioned this convention. I'll probably add to this list especially since I'm still learning the whole system.

Citybus