This is an octet-string bitmask, paired with sfpsChassisRipFPPortMask in the same table row, that shows which inband (internal backplane/stacking) ports connect to a discovered peer chassis identified by its MAC address. An admin can compare this to the front-panel mask to tell whether a given neighbor was learned over the external front ports or over the internal stacking interconnect. For illustration, a mask with only bit 2 set would mean the peer chassis was discovered solely via inband stacking port 2.