This read-only integer states which of the four hardware groups the port summarized in this row belongs to, so link-state and error fields elsewhere in the same table row can be attributed to a specific physical microhub. An admin checking why a particular jack shows psmLinkState down would first read psmGroupIndex to confirm which TP3000 or RP3000 card holds that port. Group 2, for instance, would point the admin to the second microhub module rather than the first or third.