Allow arguments/parameters to be passed to variable definitions, effectively creating user-defined functions.
For example, suppose that I want to define the function TagSum, which adds the values at all events in a time range. Then:
TagSum(tag, start, end) = TagMean(tag, start, end) * EventCount(tag, start, end)
https://pisquare.osisoft.com/thread/7591
Interestingly the PI on VAX/VMS system which was available in the 80s - had an analysis system not too different from the PI AF Analysis system we have now - and it had user defined functions that could be called from other analyses.