A faction claims land by chunk, dependent on how much power they possess at a rate of 1 chunk per power or at a set amount per faction member when in DTR mode. Within its territory, members of the faction can exclusively build without interference of non-members (configurable through the /f perms command). Your faction's territory is where you can build your base, set your faction's home location, create warp points, and more.
Want to see the boundaries of the chunk in which you're standing? Use the /f seechunk command.
Power
Every player has a quantity of "power." A new player typically starts with 0 power and slowly gains power over time by playing on the server. Dying takes a way a set amount of power.
When a player is part of a faction, their faction's power is calculated as a total of its players' power. Power is how factions acquire territory, or raid another faction. A faction can claim territory as long as its power is greater than its current territory claims.
If, through player death or players leaving a faction, territory count is greater than power the faction can be raided by enemies who can now destroy blocks in their enemy's territory and maybe even claim territory away from the faction.
DTR
An alternative to power, Deaths 'til Raidable is a mode in which land claiming does not depend on power but instead factions can claim a certain amount based on faction membership. Every time a faction member dies, the faction's DTR value decreases (typically by 1) and once it hits 0 the faction's land can be raided and looted.
The server will announce when your faction is and no longer raidable.
