One of the biggest differences is that firmware versions v0.17 (the first version of the firmware installed on E2 frames released to the public) and v0.24 only had semi auto, classic mode and training mode - there were no ramping modes originally.
Firmware version v1.10 added PSP ramping, and Millennium ramping. Both of these had 15.4bps max rate of fire, and required three shots in semi auto at a rate of 5pps or higher in order to actvate ramping. The only difference was that PSP ramping remained active for one second after the last trigger pull, whereas Millennium ramping would revert back to semi auto as soon as you stopped pulling the trigger or the trigger pull rate dropped below 5pps.
Firmware version v1.20 added the latest version of Millennium mode, that required a trigger pull rate of 7.5pps or higher to activate ramping.