How do you sign a treaty with "Norsca"?
Norsca is largely divided into seven major "Kingdoms". They're each ruled by a King, with several Jarls (high ranking nobles) below them controlling territory and holding land. The Jarls then have warriors, clan members (anyone who doesn't fight is considered a "peasant") and Thralls (slaves) below them in that order. The seven kingdoms are the Graelings, Skaelings, Bjornlings, Vargs, Sarls, Baersonlings and Aeslings. Norsca can then be divided into the Northern Kingdoms and the Southern Kingdoms.
The Northern Kingdoms are the Vargs, Aeslings and Graelings. The Southern Kingdoms are the Baersonlings, Bjornlings, Skaelings and Sarls. The Northern Kingdoms are closer to the Wastes and therefore deal with the Kurgan more often, and are significantly more Chaotic as a result of their proximity to the Wastes. They are more devout, more war mongering and violent, and more dedicated to the gods and their ideals so they raid a lot, which includes raiding the Southern Norscans. The Southern Norscans trade about as often as they raid, and are generally less violent and devout than their Northern Cousins. Page 140 of Tome of Corruption says this about them: "They raid when necessary for survival, but are more interested in acts of heroism and adventure over the carnal slaughter embraced by their kin." Unfortunately they still join the hordes of Chaos when they go south because they'll be annhilated if they don't.
Here is a map of the territories:
I say Kingdoms, but that's mostly because they're ruled by kings. They are actually called "Tribes" by Tome of Corruption, although I'm not sure about the terminology because as far as I know if a Tribe has a King it's a Kingdom not a Tribe. The settlements they have are not always permanent, they're generally semi-nomadic and move wherever there is land to cultivate. The Southern Kingdoms are willing to sign treaties and trade with Marienburg and Kislev, because they always have someone else to fight. Norscans are not united, and they fight each other very frequently, so they're never out of targets.