Dominate monster affects monsters, obviously — but my question is, what exactly counts as a monster? 5e Creature types include Aberration, Beast, Celestial, Construct, Dragon, Elemental, Fey, Fiend, Giant, Humanoid, Monstrosity, Ooze, Plant, and Undead.

"Humanoid" is what gets me. Of course, dominate monster would work on abberations, beasts, fiends, monstrosities, and the like. But what about things like Giants? What exactly is the difference between a giant and a humanoid? Humanoid's definition is generally "having an appearance or character resembling that of a human." (Webster's), i.e., 2 arms, 2 legs, bipedal, etc. In that, a "giant" is technically a humanoid — so would it be effected by dominate person or dominate monster?

Alternatively, consider the Treant — it's classified as "plant". Would dominate monster work on it? I mean, you can't cast dominate monster on a normal tree — because it's a freaking tree, not a monster. In addition, treants have 2 arms and 2 legs and walk upright, so they would technically also fit the "humanoid" description.

One more example — celestials. Celestials are generally of good alignment, so would they be classified as monster? Take the unicorn — it's alignment is lawful good. Monsters are generally interpreted in most cultures as "evil". As such, would the unicorn be a monster, even if it only wanted to help people and spread joy and whatever? Aternatively, the Planetar — it's a humanoid, but it's classified as Celestial. I hate to sound like a broken record, but again, the planetar looks almost exactly like a human, apart from size and skin tone. Would dominate monster effect it, or dominate person?

This is probably an obvious question, but it's still confusing that a spell entitled "dominate monster" would also work on a potentially good-aligned humanoid creature.