Need help clarifying how PMKID is used in WPA2-PSK.

I have successfully performed an attack that utilize the "PMKID attack vector of a WPA2-PSK network" (the one discovered by author of Hashcat). I tried this against a range of APs, where the only vulnerable AP I found, was configured with "WPA2-PSK", with both a 2.4 and 5 GHz network. I understand that PMKID is used for "reducing the delay when roaming between APs" - but does this apply to roaming between the 2.4 and 5 GHz networks under same AP? I also understand the the PMK is equal to the "secret" in PSK networks, and that the PMKID requires this PMK (in additional to MAC etc.).

As this is kinda unclear to me still, i apologize for poor explination and appreaciate all feedback :)