From SA Mafia Wiki
A Miller is a town role in which a player's alignment is town, but they wrongly investigate as scum to Cops. Typically they are told as such, but occasionally the player does not know their miller status until they die and are flipped.
One reason for being a miller can be to balance out an otherwise very powerful town role. Another reason might be for flavor reasons if the game is a flavor heavy one. Other times it is simply a useful role to balance out a game, as it can weaken the town slightly without giving the scum new powers.
Miller is a common scum fakeclaim, as it easily explains a scum investigation result, and there is no way to be certain if someone is truly a miller or is really scum. Millers are often seen as a dangerous player to have at end game, as there will often be doubt as to whether they're really scum or not; this can distract the town from hunting scum.
Some argue that a Miller claim should always be tested as soon as possible (via execution or vig kill) to avoid a continuing debate about whether or not the Miller claim should be tested. This allows the town to get the Miller debate out of the way and helps prevent tunnelvision by some players. But others say that a miller's actions should be judged based on merit, just given extra-scrutiny. Still others refuse to ever vote a miller on Day 1 out of principle.
Infrequently, a Miller may only investigate as scum on certain nights, for example, "even number nights," or "nights where you used your power."
A miller that, when died, flips scum despite really being town-aligned is known as a Death Miller.
The standard in the SA Mafia community is that millers should (and do) claim on Day 1, sometimes even in their first posts, since if a miller does not do so, and is later investigated as scum, a town is probably very reluctant to believe them. However some argue a miller should restrain from claiming since it creates a distraction to hunting scum. This may especially be true if the miller has a powerful role that might hurt the town if they removed the miller from the game early, or if the miller knows they can play in such a townie way that a cop wouldn't want to waste an investigation on them in the first place, making their miller status moot.