Pool-Name
From FreeRADIUS Wiki
To assign a user an IP from a pool you simply need to have a Pool-Name Attribute (Keep in mind that it is a CHECK item, not reply) in the required configuration file, which is either in files(users), sql or any other type of configuration schema.
DEFAULT Pool-Name := main_pool
Fall-Through = Yes
You will of course need to have setup one of the IP Pool modules for this attribute to have any effect at all.