This is the aggregate-port numbering on the peer side of this compute cartridge's physical port endpoint's link, populated when that peer port is itself a member of a port channel. An admin verifying that a port-channel bundle on the cartridge is correctly matched on both ends checks that this peer numbering lines up with the local AggrPortId scheme, since a mismatch here is a classic cause of a port channel failing to bundle correctly. A value of 0 means the peer side is not part of a bundle; a nonzero value like 2 identifies the peer as member port 2 of its own port channel.