Protocol ID / Organization Code (3 bytes) (SNAP)
Protocol: SNAP SNAP PCI
Field: potocol ID / organization code
Length: 3 bytes
Contents: protocol suite ID.
Originally, LLC's upper layer protocols were supposed to be identified by LLC's 1-byte SAPs. Example: DoD IP was identified by SAP=0x06. But after LLC being standardized, new protocols were introduced (as ARP/RARP, SLIP etc.) that couldn't be addressed by SAPs any more (only 6 bits out of 8 SAP bits can be used for protocol identification).
Therefore, SNAP was created. It uses the same 'type' values as Ethernet II does, but the EtherType field now was preceeded by a 3-byte field for protocol familiy identification (protocol ID).
In case of DoD IP (type=0x08.00) the protocol ID is set to 0x00.00.00.