CDB - Command Descriptor Block

First Byte -- Op-code  ( group code + command code )
Last Byte -- Control byte (NACA, Link)

 How does a target know how many bytes it should handshake ?  Group Code.

When a transfer length is required of a CDB, it usually specifies that a number of blocks will be transferred.

When a parameter list length is required of a CDB, it specifies that a number of bytes will be transferred from the I to the T.

When allocation length is required of a CDB, it specifies that a number of bytes will be transferred from the I to the T.