What is the fixed-on interval with regards to stepper motors?
The fixed-on time is the time during which no current regulation is done (other than absolute current limiting). This allows for faster acceleration at the beginning of each pulse, if the motor is not in inertial motion already. 

Fixed-on time ends early if current reaches desired value before time-out, and this is the ideal case for a running motor. 

A balance must be struck between how quickly the motor spins up from a standstill, and how gently the acceleration pulses impact the mass while the motor is in inertial motion.

For large / high horsepower motors, standard lookup tables are often available. For small and hobby motors, it is pretty much an experimental science. 

One way of looking at FoT is as a simplified PID mechanism.