With the 744 ECS and checklists in mind, the three air conditioning packs are usually set to either "OFF" or "NORM". However, there are two other notches on the dial for "A" and "B", and I'm wondering what they are.
My understanding is that the "A" and "B" systems are just another example of redundancy in the cockpit. For example, in "NORM" mode, the pack controller automatically selects either the "A" or "B" system "on alternating flights" (got that from an AOM, which I found to be pretty interesting), but you can also use the other two modes to specify either system as the primary and have the other stand by as a backup.
Is that really all it is, though? Why would one want to (or need to, aside from a known faulty pack) submit their own preference for A vs. B and not have the system make the selection automatically?
And no, not that A vs. B...