Factions are how we gate some of the features of Tlessa. We use Quests as well to gate features and advancements.
As you have seen from Quests documentation, there are a ton of quests that start on surface to give you access to other planes with specific instructions on how to complete them.
If you are missing a required item to be able to hand in and be teleported/traversed to the NPC, there are even explicit instructions on how to access that NPC.
As you can see, to the right or bottom for mobile, there are 5 planes that you can gain factions on.
Factions are like notoriety. You kill monsters on a given plane, any monster, to increase the faction points and level them up
Each monster you kill gives you 2 points from level 0 – 1 and 1 point after level 1. Each Faction has a total of 5 levels.
There are quests that require you to level facts, and even a quest that give you a faction quest item to give you more points per kill.
It is suggested that players do not manually try to grind these out, and instead use exploration, which give you points per kill and bonus points along the way.
Factions also give you uniques, worth 10 billion gold per level up. Meaning every player can get a total of 25 uniques at a valuation of 10 billion gold each.
The Queen of Hearts (late in the game) can let you buy 50 and 100 Billion gold uniques as well as re-roll and move the enchantments to other gear.