If you are looking for a bit that is true for the first scan of the processor and then false, there is no system tag that has this characteristic. However, you could create one by setting a bit high in a Power-Up task (which runs once before the first scan of the main program) and then setting the bit low at the end of the main program cycle.
More information on logic blocks and execution style can be found in the MAPware-7000 IEC61131 Programming Guide.