What determines feed rate ? Is it based on the software that is loaded in mach3 to produce the g code?

Yes your feedrate is set in your code, for example if your code commands a G1 move it wil also have a feedrate, something like G1X100 Y20 F100.
The F100 is the feedrate which would be inch per min or mm per min depending whether you were using imperial or metric.

Hood