First number is tractor series, next three are engine hp.  Then first letter is spec level (R being higher spec than M or E).  Second letter is special specs (T for tracks, E for Electric, H for high crops, N for narrow, F for orchard/fruit crops and V for vineyards).

So a 6210RE is a high spec 6 series tractor producing 210 hp and with the flywheel generator; a 5100GF is a medium/low spec 5 series orchard tractor producing 100 hp.