https://wiki.mikrotik.com/wiki/Manual:IP/Hotspot#HotSpot_Setup
Address pool for HotSpot network, which is used to change user IP address to a valid address. Useful if providing network access to mobile clients that are not willing to change their networking settings.
I am always set to None. When access points of other vendors are connected to Mikrotik, then clients fall off on them, if a pool is used.