There is a quest at lvl 75, have to kill some RB and more..
Then you can pick a class (LE and DE can't sub each other, and you can't sub simular classes DA/TK/Paly or HE/SR/PR. No one can sub OL or WS).
Then you get a completly separate char (only name, looks and WH are shared) and you can switch between your's chars in town.
Afer your's sub gets to 75 you can repeat it all ower.
EDIT: nm, read the website some more. basically its so if ur a gladi and get ranged, u can switch to ur nuker sub and nuke ur pker when they're not looking?