802.11bn indeed is not a standard, but the 11a, 11b and 11n are.
802.11n is the first dual band standard and successor of 802.11a (5GHz) and 802.11b (2.4GHz).
What you see happen in some places is that the a and b are added to indicate the band:
802.11an = 802.11n on 5GHz
802.11bn = 802.11n on 2.4GHz
This probably is not compliant to the official naming, but I have seen it used in multiple places.