What does flight categories "C", "G", "T" etc. mean?
I got the answer.  'C' is commercial, 'G' general aviation, 'T' taxi, etc.

Update:

http://www.faa.gov/air_traffic/publications/atpubs/fss/AppendixA.htm, under section "Type of flight", it lists


  S  if scheduled air service
  
  N  if non­scheduled air transport operation
  
  G  if general aviation
  
  M  if military
  
  X  if other than any of the defined categories above.


But types C and T isn't there.