The letters: A, B, L, C, R designate the autopilot in use. On the 37 there's two: A & B and on others there may be three: Left, Center, and Right.
Normally the pilot flying will use the autopilot on that side of the plane, A for left or B for right, during cruise.
When using the autoland modes both or multiple autopilots are engaged simultaneously to ensure safe, continued operation in the event of a failure.
Very simply put there are two modes of autoland: Fail Passive and Fail Active.
In the Fail Passive mode you have two autopilots and if one disengages, due to a fault, above a threshold altitude AGL (may be 50 or 200 ft) the pilot flying is to disengage the remaining autopilot and perform a manual landing.
In Fail Active with two autopilots and a HUD or three autopilots (depending on the certification level of the operator) the remaining autopilot(s) will be allowed to take the aircraft to a landing with close monitoring by the pilot flying. I know of no aircraft certified for single autopilot landing. (that, of course, doesn't mean there aren't any)
Don't confuse the autoland modes with the categories of IFR landing, I, II
, and III a, b, and c. The requirements are different for the autopilots and the operators and the national certifying agencies also have their own certification levels and requirements.
Good. . . There's a whole lot more information about specific autopilots and navigation interfaces and just about every aircraft out there has its own specific idiosyncrasies. Even though different aircraft may have the same box installed the software will be different for each application.
After working on and helping develop numerous autopilots over the last 38 years I feel that my knowledge only scratches the surface. I'm amazed to see what's happened in the avionics world. From my first autopilot on the C-47 to the SP
-1 on the 141 to the Honeywell and Collins on the 737NG, some things remain the same but many many more have changed. Whether for the better or not is for others to judge.
[Edited 2007-12-19 15:33:33]
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533