Inverter's with two AC input ports, one for grid, one for generators, often allow a wider AC frequency range for the generator AC input port to allow for sloppier generator frequency tolerance.
If the inverter does exporting to AC input, backfeeding, it is usually blocked for generator port to avoid pushing power to a generator. Some units have an external CT sensor for grid port to allow referencing back feed to main breaker panel grid input to allow export supplementing to main panel breaker loads while avoiding back feeding to utility grid.
Other than these items you can put grid on gen port.
Putting generator on the frequency range restrictive grid port may prevent inverter from sync'g to generator. Sometimes the default freq ranges can be adjusted by user to widen out the acceptance range on grid input.
Widening frequency acceptance range can increase the inverter synchronizing lock time a bit because it has to try a wider frequency matching range in the initial lock search scan range. It also can increase the anti-islanding release time for the inverter.
