On a SCSI disk following Inside Macintosh V, the first block (block 0) contains the driver descriptor map which is the first data read by the boot code in the ROM and contains stuff like number of blocks, block size, number of drivers and location and size of the first driver. The boot code then loads this driver into memory and executes it.
The partition map starts at block 1 and each partition map entry with information relating to it occupies 1 block.