The blocksize of the tape device.
Tape-Blocksize

This value specifies how many bytes are written to tape or read from it with one system call. Usually this value is at least 512 or a multiple of it. It is not very important if the blocksize is set to 2048 or 1024. The main thing to keep in mind is that if there is a minimum, it should be respected (e.g. 1024 on AIX), otherwise media space is wasted.