A CBM/GEOS block is a logical region of a file stream. It always consists of 256 bytes of data (typical 254 bytes of user data preceded by a 2-byte file link). A file link is normally a track & sector pair which specifies the next block in a file stream. The exception is the last block of a file stream; in this case the track# is zero (invalid) and the field for a sector# holds the #bytes + 1 that are valid in the last block.

For CP/M disks, an analogous concept is the (file) allocation unit. However its size varies: it is 128, 256, 512 or 1024 bytes depending on format (known as 1, 2, 4 or 8 records). A (file) allocation unit does not include a file link. Instead the file chain and bytes-in-last-unit values are stored in directory entry(s).