JAR requires trip+5%+taxi/apu+alternate+final reserve.If you cannot load all that due to mtom restriction or performance etc you may reduce the contingency (that's the 5 % ) down to an agreed minimum figure.This is decision point procedure and the CFP must be annotated as such.A "decision point"is nominated at which the crew will decide whether to divert to the agreed en route alternate or continue to destination.You would need to have on board at that point fuel to dest+5% of THAT fuel (ie a lesser amount than the "full"contingency+alt+final reserve.
If you cannot even load up to the required fuel figure using minimum contingency you must plan on departing to a closer airfield.At a realistic descent point for that airfield the crew may "re-plan"to the original destination if they reckon they can get there with alt+final reserve.That is "In flight replanning " or "re-clearance".Yes there is still a decision involved but the diference is all about the planning stage and less about the actual flying.Both involve much fretting over headwinds and begging of direct routings.
There are a few complications and lots of discussion about eating into your alternate fuel etc but thats another can of worms.
Remember that you could theoretically use all your contingency fuel on taxi out (not quite sure how but ,hey!) and legally get airborne with just trip+alt+FR.