G94 is feed per minute mode. G95 is feed per rev of the spindle. When you write a G code program, you can specify which mode to use in the first line of the program and it will stay in that mode until you change it. If you enter a feedrate into the MDI box the mode will be whatever was previously specified.
During testing of your machine, I would suggest you stay in G94 so if you enter a feed rate of 50, the axis will travel at 50mm per minute. If you were in G95 mode the axis would try to move at 50mm per rev which would be much to fast.
It is not necessary to enter a feedrate into the MDI box everytime. If you have already entered F50, it will stay at that rate unless you specify differently. When you start up Mach each time, the feedrate is defaulted to a low value (I think 6mm per minute)
The maximum feedrate is dependent upon the velocity as set for each axis in motor tuning (your rapid feedrate)
Neptune