read AC 121-22A for the number used to separate GA aircraft and transport category aircraft, then read
http://en.wikipedia.org/wiki/Transport_category
for a better understanding that part 25 and 29 apply to aircraft over 12,500.
plus 23.3 limits what a normal category aircraft is,
23.3 Airplane categories.
(a) The normal category is limited to airplanes that have a seating configuration, excluding pilot seats, of nine or less, a maximum certificated takeoff weight of 12,500 pounds or less, and intended for nonacrobatic operation.
so, if it is not a normal category aircraft the maintenance is directed by the type of certificate it operates under.