The ISO 9660 primary volume descriptor begins 32,768 bytes (32 KB) into the disc. If present, an Apple partition map begins 512 bytes into the disc; if there is no partition map, the header for an Apple HFS partition (known as a Master Directory Block, or MDB) begins 1,024 bytes into the disc.
The ISO 9660 primary volume descriptor begins at a specific point on the disc, 32,768 bytes into it. If applicable, an Apple partition map starts 512 bytes from the beginning; if not, the header for an Apple HFS partition (Master Directory Block) commences 1,024 bytes in.