This value encodes which specific radio this entry refers to -- on a wireless controller it packs the AP's index into the upper 24 bits and the radio number into the lower 8 bits, while on a standalone fat AP it's simply that radio's interface index. An admin decodes this composite index to identify exactly which physical radio, on which specific access point, a given entry's settings or statistics apply to when managing many APs from a central controller. A value where the upper bits decode to AP 5 and the lower byte decodes to 1 would identify this entry as referring to the first radio on access point number 5.