The Knight variant of the Noble background (PHB page 136) gives the following feature:
RETAINERS
You have the service of three retainers loyal to your family ... one might be a majordomo ...
I can't find any other mention of a majordomo in the PHB, so what is special from a D&D perspective about a majordomo vs any other retainer?
It might be worth noting that this feature seems to have said squire once upon a time if this question is correct, and a squire is a specific NPC.