What is difference between passing an input vs output data in a script task?

Not a huge difference.

I use it mostly to reduce the amount of script tasks in a process, more specifically when I want to store 2 PVs but need the output of 1 PV in order to calculate the 2nd PV. In the Outputs tab, you can access the data from the input by using the ac! notation (Activity Class Parameter).

docs.appian.com/.../Process_and_Report_Data.html

Hope this helps!

Adding to what [mention:f5e81b8502fa4e77ad12697686a4fc56:e9ed411860ed4f2ba0265705b8793d05]  already explained, it is important to understand that all assignments in the input are evaluated in no specific order, but before any assignments in the output. Assignments in the output also evaluate in no specific order.

Apart from what has already been explained, please know that you cannot use any input variable to derive the value of another input, and the same goes for the output.