The stress ratio R is usually defined as minimum stress value divided by maximum stress value, which leads to values between -∞ and +1.
The software is using a value between -1 and +1 instead and it defines the stress ratio using the following definition:
Using this definition R = 0 can be used for pulsating loads between zero and a positive or a negative value. For alternating load R = -1 and for constant load R = +1.
The stress ratio can be defined for tension, bending and torsion independently. For a rotating shaft the stress ratio for bending should usually be -1. If the stress ratio for torsion is not given, make a calculation with two cases: constant and pulsating for example. It depends on the current loading which case is conservative.