Cards are numbered by the number of outside lines that they provide, followed by the number of internal extensions.
So a 412 card is 4 lines and 12 extensions. The 412 should be followed by TDL, which are the single-pair 4400-type extensions used in a Merlin Magix.
An 800 is 8 lines, no extensions. The 800 should be followed by GS/LS-ID for Ground Start/Loop Start-Caller ID Capable. It could also be followed by DID, which are analog Direct Inward Dial trunks.
All cards go into the slots of the carrier, and the processor takes care of "connecting" everything together.