In JAA land we are required to declare a PAN PAN status when there is a good chance we might go into final reserve fuel and we have to declare a MAYDAY when it is assured we will go into final reserve fuel.
However i can still enter the aircrafts low fuel state without any of the above being applicable. It is possible to plan in such a way that the total reserve fuel (alternate and final reserve fuel) is less than the fuel required to keep above the low fuel status. If i do enter low fuel status i have to do the non normal checklist LOW FUEL which places some restrictions on me in terms of climb and descent rates as well as turn rates. In that case i would advise ATC that we are low on fuel, however refuse to declare an emergency.