Knights and Knighthood

[GypsyComet]

Sector and Subsector Dukes can apply the the Emperor for the right to found an order for their demense, and the founding Ducal title serves as Grand Master. The Emperor has ultimate fealty of the knights, but the Orders are pretty independent of each other. For some, it gives them a place in the hierarchy, others, it's the ability to do a job for the Good of the Imperium (in theory...). Of course, getting a knighthood one of the Archducal or Imperial Orders is a goal.

That's sort of the role Knights Retainer fill. All Dukes have the ability to appoint such, and they are effectively Knights within the sphere of influence of their Duke. Just don't appoint too many. The Imperium expects most Knights Retainer to be put up for full Knighthood and be worthy of that, so a House with a lot of them attracts attention...

Past material has already defined Orders of the Domains, and their memberships likely go back to before the Civil War when the Domains were still strong.