The 2mt is to align the centre of the chuck to the centre of the rotab
You'd normally do this with a dial gauge (mounted in the milling chuck) by tapping the rotab-chuck until the job is centred.

If you just want to quickly centre the chuck just drop a length of 1/2" or 16mm rod down the chuck until it centres in the taper then tighten the chuck (admittedly, turning a short 2MT onto the end of the bar would make a better job of it).