When doing arrayPolar (apart from being a really fiddly and annoying tool), you cannot enter a negative value for the Z offset.
There is also insonsistency around terminology. When defining the angular fill you have to define the total angle (unless you specifically choose step). When defining the Z offset, it defaults to the step offset rather than the total.
These two are exactly the opposite of what I’d normally want. Either way, I think these options would be far better dealt with via a pop up dialogue that allows you to amend all settings and preview the results. I find myself having to try it, realise I chose something wrong, redo it. Over and over again.
BUT, that the Z step doesn’t allow a negative number is a pretty basic issue.
Here in Windows Rhino anyway, if you have set Preview=Yes, you have a preview and can change the values as needed before hitting Enter to confirm. True that once you are in either Step or Fill angle mode, you cannot switch back to the other one; and yes, being able to define the Z offset as a total rather than a step would be good.
The disallowance of a negative value for the Z offset looks like a programming oversight to me.