t1 = area is level 68, and it scales up from there at +1 level per +1 tier.
Low tier = t1-t5.
Mid tier = t6-t10
High = t11-t15
Map mods are based on the low/mid/high thing. E.g. a low tier map cannot roll “players are cursed with vulnerability” (iirc), as that’s a mid tier mod. Map mods add to the quality and quantity of items dropped, which is why you spend currency on them. Rerolling of low tier maps should be minimal, done out of laziness on midtier (if a map rolls with something bad like your reflect opposite you’re typically better trying to vendor for an alch+ but hey if you have the currency. . .), and is usually done heavily on high tier I understand it.
Each unique “clear” (defined as: low tier “kill boss of magic map”; medium “kill boss of rare map”, high “kill boss of rare + corrupted map”) gives +1% chance that map drops on a given map will be +1 tier. Bosses have a decent chance to drop any map, and for it to be +1 or +2 tier, by which I mean “I once ran like 15 straight maps and they didn’t drop any maps at all fuck you RNGGGGGGGGGG wait j/k ilu”.
The really heavy map stuff comes from shaper/elder influence on the map itself. Zana’s big contribution is that she gives a bonus to quanity/quality based on her level (+1% per level) and you can additionally add special mods in addition to whatever other mods a map has, for a price of Chaos orbs or higher. E.g. you can add Beyond to any map for 3 or 4 c (can’t recall off hand), and as Beyond is also a mod you can roll on a map you can double Beyond.
Also wild zana = Inception, aka free map run! With occasionally brutal victory conditions.
Map pools can be hard to maintain. Low tier is easy. Mid tier is not as easy especially as you go up to the top of the tier. Top tier is not easy at all. I think it does help once your Atlas gets decently filled. I’m currently at 10+ for each of the mid tiers and I’ve been running them steadily, which is something ( I have run perilously low on Alchemy orbs twice now, though). I have never run a high tier map, but I am going to soon.
Oh, maps have variants of the bosses you face in game. The variants come with increasingly nasty abilities as you go up in tier, as you might expect.