They have unique buildings that effect the area around them and prevent resettlement until they are destroyed. This is the core of your campaign progression. Destroying settlements and defeating armies here causes Devastation, which can be turned into Marks of Ruination with the herdstone Ritual of Ruin.
There will be many opportunities to increase these caps as you advance and expand. Much like various regiments of renown, you can only have a certain number of each type of unit in your Beastmen armies.
