XPLMNavFlightPlan

These enumerations defines the flightplan you are accesing using the FMSFlightPlan functions.
An airplane can have up to two navigation devices (GPS or FMS) and each device can have two flightplans.
A GPS has an enroute and an approach flightplan.
An FMS has an active and a temporary flightplan.
If you are trying to access a flightplan that doesn't exist in your aircraft, e.g. asking a GPS for a temp flightplan, FMSFlighPlan functions have no effect and will return no information.