These are the latest roms for the N910T The number in the middle is the RP bit. OK2 has a RP of 2. So you can only flash roms with a RP of 2 or higher. So if your phone is on OK2 you can not downgrade entire rom to any rom with a RP of less than 2. So if phone is on OK2 you cannot downgrade to any other rom since all other previous roms have an RP of only 1. However if your phone is on OG2 you can downgrade all the way to NK4 since they both have RP of 1.
This is the same with exynos phones but there is a big difference. On a qualcomm phone you can downgrade a modem even if the RP is lower. However, on exynos phones if you flash a modem from a rom that has a lower RP it will cause an unknown baseband and null IMEI. That is why people trying to downgrade modem on Canadian S6 edge plus and Note 5 are having issues with downgrading to unlock. There was never a rom with older security that has the same RP as the newest rom with the new security.
Also the letter after the RP bit signifies an android version update. As you can see above NK4 has an A after the RP bit which was 4.4.4. And the 2 after that have a C after the RP bit which signifies the change to 5.0.1 and OK2 has a D after the RP bit which signifies the change to 5.1.1
All models have the same format so you can always tell if you can downgrade or not by simply comparing the RP bit of the rom you want to flash to the RP bit of the current rom on the phone.
And if you look at at&t or verizon pda versions you will never see a model with roms that have the same RP. They call this the so called locked bootloader. But its real name is ROLLBACK PROTECTION.