If you cast summon greater demon, it says
You utter foul words, summoning one demon from the chaos of the Abyss. You choose the demon’s type, which must be one of challenge rating 5 or lower, such as a shadow demon or a barlgura. The demon appears in an unoccupied space you can see within range, and the demon disappears when it drops to 0 hit points or when the spell ends.
It says demon, but what creatures qualify? Clearly, it’s not all fiends, because devils are different, but it can’t just be creatures under the demon section of the monster manual.
Demons are a specific type of creature, and, luckily, D&D Beyond lets you search exclusively for demons. It even lets you limit them by CR, so you can gets lists like all demons of CR 5 or lower. It is worth noting that these lists will include certain variants as separate entries; for example: the summoner variant of demons and the reduced threat variants of monsters; whether these in particular can be summoned is not entirely clear and is left to the GM.
In particular, “demon” is a Monster tag which are briefly gone over in the Monster Manual, page 7:
A monster might have one or more tags appended to its type, in parentheses. For example, an orc has the humanoid (orc) type. The parenthetical tags provide additional categorization for certain creatures. The tags have no rules of their own, but something in the game, such as a magic item, might refer to them. For instance, a spear that is especially effective at fighting demons would work against any monster that has the demon tag.
If a monster is a demon, the demon tag will appear in their statblock; for example, the quasit has a statblock that contains the following at the top:
Tiny fiend (demon, shapechanger), chaotic evil […]
This tells you that it is a fiend, but in particular it is a demon and a shapechanger for any features that rely on those terms such as the summon greater demon spell.