Under Command/Army Sponsorship... there is option "Found army".
There is field to enter "Army name" and drop menu to select base region for the army.
Is "army base" location where members of the army will be able to recruit soldiers for their units, or?
If not, what does this option affect on?
If the base region is lost, the army will disband.
If you ever wish to reassign sponsorship of the army, it can only be reassigned to the Lord of the base region.
At present, I believe those are the only effects of which region you choose as the base.