Step time is the time span of physical simulation in a single Maya frame. By default, in each Maya frame, the PhysX simulate and increase 0.1 second for updating physical contents.
Sub step is the number of steps in each simulation time span. For example, if sub step is 4, and step time is 0.1, each time PhysX simulate, it will simulate 4 times and each time takes 0.025 second.
Increase sub step can achieve precise simulation result but take more time to calculate.
Let's take a look an example, when the agents are pushed by force field, the different sub step lead to different precise level.
Sub step from left to right: 1, 2, 4