Warning! Potential spoilers ahead.
From Making Star Wars:
“[…] Another rumor I have heard a few times pertains to those red troopers I wrote about sometime ago. I have heard that the red troopers are called “Sith Stormtroopers.” Now, are they called that behind the scenes because Kylo is actually a Sith Lord now or do we have a game of synonyms at play where “evil” is the same as “Sith”? Anyways, the pertinent info here is that these red troopers answer only to Kylo Ren and The Knights of Ren exclusively. Maybe they’re just the Knights of Ren’s infantry? […]”