T105 is the parameter.
Also relevant here is parameter T106. This parameter determines whether the above safety fault can be reset using the clear faults method that OP is attempting, or whether it requires a power cycle to clear the safety fault. By default, this parameter is set to require a power cycle to clear the safety fault.
So, out of the box, if you use STO on a powerflex 525 (which you should), you will need to change at least one of these parameters to make the system work properly (otherwise you have to make sure your safety circuit is always healthy before your drive powers on, which is obviously impractical). You can either disable the fault altogether (which is what I typically do), or you can allow the fault to be reset over the network using the ClearFaults bit, so that you can reset the fault once it has appeared.