extract from the documentation page
Require Use of Radio Firmware x.xx
This setting affects the firmware upgrade process when you load new firmware for the access point. Select yes to force the radio firmware to be upgraded to a firmware version compatible with the current version of the management system. Select no to exempt the current radio firmware from firmware upgrades.
The idea behind this setting is to block clients that do not have the firmware xx this is due to the fact that some security feature require the clients to have a minimum firmware version and old firmware clients will not use this feature (TKIP and MIC are examples of these features that act this way) thus side stepping this security feature