In general, each manor is expected to field 100 militia for the army. Some manors do not have the population to support that, so they either conscript their peasants, or hire mercenaries to fill out their obligation, or beg the indulgence of their lord. So a landed knight going to war is expected to bring 100 peasant militia with them, equipped and supplied for war.
When a manor goes to way and is short, it is Earl Rodrick's responsibility to make up the difference with mercenaries.
Valka and Arne are a little different. They and their people are working as mercenaries in the employ of Earn Rodrick, helping cover the shortfall from Roderick's landed knights.