RSMB vs RMB

What is diferent wifi RSMB vs RMB?
Wifi 2,4 is RSMB, why Slave?
Router - hAP ac3.
Thank you.
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I’ve more than 20/25 ac3 still running in 7.16.2.
All are configured as standalone, not capsman here.

i’ve check 5 devices, and all the wifi master int (wifi1@2g/wifi2@5g) are all in RMB.

Is WiFi2.4 a port of any bridge?