There are four traversable blades of hell; alienation, passion, hatred, and zhushanar (“sorting into needless hierarchy for reasons of ideological benefit”).
The fifth blade, rage, was extruded over the entirety of Nylsanna's stone, the bane of reason that alters reality in nonsensical ways.

All demons are IMMUNE to poison and the poisoned condition. Unless otherwise specified, they are chaotic evil.

Demons that are equally attuned to all four blades are "full demons". Powerful full demons are normally resistant to cold, fire, and lighting.

Demons that lack attunement to the hatred blade of hell are "preserver demons". The hatred blade of hell is represented by the element of cold, which they lack any resistance to. They are IMMUNE to lightning however, and resist fire.

Demons that lack attunement to the alienation blade of hell are "slanderer demons". The alienation blade of hell is represented by the element of lightning, which they lack any resistance to. They are IMMUNE to fire and resist cold. Most of these demons can see in all darkness, even magical darkness. These demons are lawful evil.

Demons that lack attunement to the passion blade of hell are "idleness demons". The passion blade of hell is represented by the element of fire, which they lack any resistance to. They are IMMUNE to cold and resist lightning. These demons are neutral evil.

No demons lack attunement to the toxic zhushanar blade of hell.

In the stock D&D universe, the term "fiend" describes all the denizens of the lower planes. This includes the tragically noble devils (they are like, HUGE devil fans over there), who inhabit the nine hells, the infinite horde of demons (who come out of the infinite abyss sometimes, but most demons spend all their time fighting with each other), and the freakish daemons who, for whatever reason, are often persuaded by money to work for this or that group.
In this cosmology, the devils are noble because they spend a lot of time fighting demons. Demons are infinite and can't be defeated, but since they occupy infinite space the numbers that come out of there are finite, and this justifies any size demon army that you want to have, from "this one guy" to "a planetary mass's worth of them". Devils are lawful evil, immune to fire, able to be hit with lighting, can see in the dark, and during the satanic panic of 2ed were temporarily renamed to "baatezu". Demons are resistant to all those elements but aren't immune to anything normally (demons with an elemental theme may be immune to something- there's some sonic demon that's immune to sonic, for instance). During the satanic panic, these were the "tanar'ri". The daemons resist stuff and are immune to acid some of the time; other times I think it's not exactly clear. They mostly are called "yugoloths" in recent stuff, having never recovered from the satanic panic (and also having landed on a better name, and also "daemon" is literally the same word as "demon", so it was always stupid to call them that).

While I have no use for any of the stock D&D cosmology- basically every fiend is a demon, and they are numerous but never infinite, all created by Eshcamit directly or indirectly- I did mirror the mechanics of the devil's immunity to fire to provide for different immunity and resistance for the other two classes. I think this makes the damage resistance/immunity thing way more interesting, but you guys will be the judge of that.

Note that on Caligo, the universal term demon is applied as well, and they don't have these subdivisions, which was also how I ran Kylon. I don't have any notes mentioning demons on Tanna at all, and I don't recall any encounters with such creatures.