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Oops, sorry for leaving that part so cryptic!  The whole loot/gating logic was kind of a work-in-progress, so I didn't really get to polishing it up.

You're right about the first two: the major treasure for the area, and the post-boss treasure.  The remaining four are all locks.

The third one is "ability needed to navigate this area", and it doesn't have any actual implementation on the overall map.  It's more of a suggestion, but it basically boils down to "whatever you got from the previous boss".

The fourth one is "ability needed to access boss", and is responsible for the various locks that lead deeper into an area, particularly those around the area boss.  This one is usually the big treasure in an area, though sometimes it requires you venture into a later area a bit and double back.

Next is "ability needed to access area".  These are the locks in the inter-area passages.  They're the simple ability check before progressing to a new area.

The final one is "ability needed for challenge paths".  Occasionally an area will contain optional one-directional paths, usually with some loot hidden inside them.  They're supposed to be more dangerous technical challenges.  This icon shows which ability you'll need to navigate these paths, which is usually one you'll gain from a later area.

Hope that helps!