While mortals can exemplify an alignment, creatures from the Outer Planes personify them. A mortal can have the trait of being, say, Lawful Evil, but a devil is Lawful Evil, that is, it is the fundamental essence of what it means to be Lawful Evil. When someones asks what Lawful Evil is, one can point to a devil, not to say, "This is an example of Lawful Evil," but rather to say, "This is Lawful Evil."
We argue about what alignments mean for our mortal adventurers, and for them alignment can seem loose and mutable, which is often the cause for those arguments. But for outsiders, alignment becomes much more strict and important. A human, elf, or dwarf can go through a series of experiences that causes them to change their alignment. The experiences can be profound, or traumatic, or enlightening, but it doesn't change the fundamental nature of their being. But if a devil stops being Lawful Evil, they stop being a devil... right?
A recent thread about how to play an evil character (presumably in a good party) gave me an idea for a PC that is literally a devil (mechanically a tiefling) with a possible redemption arc.
Long version here.
Short version:
Where this becomes significant, and relevant to the thread title, is here:
In this particular case, we have a devil who has to do good deeds as a punishment, but is at risk of an alignment shift (probably to Lawful Neutral), which would prevent his return to Hell. At this point, he'd cease being a devil altogether. At least, I think that's what's supposed to happen.
More broadly, I feel like this probably applies to most outsiders. Celestials have to be Good, and angels in particular seem to be all Lawful Good. If an angel falls to Evil, they become a devil, as with Zariel. Modrons must be Lawful. Slaadi are Chaotic. And so on.
Alignments might be more tightly enforced for some outsiders than others. But it leave me wonders, what exactly does happen when an outsider, particularly one with stricter alignment enforcement, changes their alignment? In my particular case, he'd probably just remain as a mortal (or immortal) tiefling, retaining but a few shreds of his devilish power.