The store page suggests this game's content in procedurally generated, and I'm not sure what that means.

[Chough]

To put it directly, is Rain World a roguelike or roguelite in any capacity? Or are procedural techniques being used in a different way, maybe making enemies learn and evolve over time? I'm not clear on how this game was designed.

It's an important point for me, and it'll make or break whether I buy the game or not. I absolutely can't stand game assets bolted together with RNGs instead of a fully designed game with a clear, curated experience. I don't even like Dead Cells, despite all the press about it being a roguelike for people who hate roguelikes.

If there's anything governed by an RNG, I don't want it. On the other hand, if it's not randomized, I'll spring right for it.

[Krоvososko]

Map in this game is handmade and really pretty,if you into technogenic landscapes.I have no idea why is there "procedurally generated" tag.

[MeeperCreep]

Unfortunately yeah, this game has a lot of RNG. Thought it is not really considered a roguelike, it's more of a survival exploration game that is intentionally cryptic and unfair at times. There's no permadeath, but death does have consequences that might halt progression to other areas.

Game is really punishing, but there's fun to be had once you get the hang of the clunky controls and how the whole ecosystem functions. But it's up to you if you wanna try it out.

[Nen☆niN]

The creature spawns and their behaviour are influenced by RNG*. Map is not. The most "procedural" thing in Rain World is it's animation.
*This is not to say that creatures are acting randomly, they usually behave mostly the same, but some RNG is still involved

I wouldn't call Rain World a survival game in any capacity, but exploration is a warranted description. Even so I'd call Rain World a platformer first and foremost.

[CandleSign]

RNG is in a way what makes things around you alive. The world itself is always the same but creatures might do different things; what makes people think procedural generation is involved is for example that their location seems random, but not at all, they may just decide to wander differently while you're not around. It's more simulation of free will than pure randomness.

[Krag]

It's not so much RNG as much as fauna being somewhat unpredictable. It's an ecosystem that doesn't revolves around you so if you're passing by and they're hungry then you'd better think fast.
It's honestly the best aspect of the game even if it's also a source of frustration at times. It forces you to learn and adapt on the fly.

[Heikai]

The unpredictable nature of the creatures is the best part of the game.

[Goblin]

What's procedurally generated, which the devs did a GDC talk on, is creature animations and behavior, specifically the interaction between both of those. The game doesn't have a sheet of animations or a 3D model that gets animated with predefined animations made through mocap or by an artist, but instead just defines limbs and tells the creature how to use them.
Think of it a little like those robot dogs made by Boston Dynamics, though a lot less refined because it just has to navigate a 2D platformer instead of the real world.

Also to a lesser degree, it doesn't generate random types of creatures, but each creature is generated as part of its species. For instance it spawns a green lizard, but this particular one is slightly bigger and more hostile so other ones are more scared of it. It's not super noticeable usually, but it does influence everything's behaviour.