Essential traffic means other aircraft which the controller must keep
you clear of. In class C airspace, for instance, IFR and VFR flights are
permitted, all flights are provided with air traffic control service.
IFR flights are separated from other IFR flights and from VFR flights.
VFR flights are separated from IFR flights and receive traffic
information in respect of other VFR flights.
In class C airspace a VFR flight consitutes essential traffic (i.e. must be separated from)
an IFR flight but not a VFR flight.
VFR/VFR flight separation is left to the aircraft but known information
about VFR flights is passed to other VFR flights.