Tag identifying a settlement NPC rather than a mob, marking them as not intended prey for crawlers.
Dungeon inhabitants are tagged by role, and the white tag marks an NPC as opposed to a mob. Dante leans on this when he reassures Justice Light about their reassignment to The Eighth Floor: because the castle they've been posted to sits in a settlement rather than a boss room, the two of them will still be white-tagged.
He repeats it twice in different words — they're still tagged as NPCs, not mobs — because it is the strongest comfort he has. It is not the same as being safe, and he knows it; he tells Justice Light outright that they will not be protected.
The status distinction Dante uses to argue their new post isn't a combat role.