Possible level/story jumps....really?

[Lokier]

you mean if it applied to a class in a role you levelled up? like say paladin is 60 you want to get war to 60 to be flexible?

Yeah, so say you've met the requirements by leveling ninja, WHM and DRK to 60, but you want a PLA at 60, then you can use the boost on your PLA (provided you've got both required classes at 30)