Straight from the manual:
http://wiki.mikrotik.com/wiki/User_Manager/Users
Pool name. If not blank, user will get IP address from this IP pool on successful authorization;
Group. Sent to Radius client as Mikrotik-Group attribute. Indicates group (/user group) for RouterOS users and profile for HotSpot users. See Radius client documentation for further details, search for "Mikrotik-Group".
Address list. Sent to Radius client as Mikrotik-Address-List attribute. Used only for PPP (not hotspot) - indicates to which "ip firewall address-list" should the remote address be added.
That last bit might be outdated. At least with other RADIUS servers the address list attribute works fine for Hotspot users.