That is "Traitor guard" in most cases. Not the typical coal miner turned devout follower of Khorne.edit: On another note Charon, some chaos warbands absolutely carry around their own hordes of cultists, wordbearers and alpha legion being the most obvious, but theres also the crimson slaughter and like the black legion of iron warriors
We see 3 groups most of the time: Cultists (that is civillians), Traitors (that is PDF, former guard, merchenaries,..) and Slaves ("Chapter servs", ex-navy, workforce,...).
They all serve Chaos in one way or another but they are different. Sometimes it is hard to tell them apart.
Take Gaunts Ghosts as an example.
The Bloodpact. Cultists or trained Soliders? Would you pick an IA army list to portrait them or CSM Cultists?
Sons of Sec? Then you have the people of Ferrozoica.