PREVIOUS returns a record, then .Running_Balance pulls out one field of it.
E.g. UPPER($A) is just a function that calls $A.upper(). And $A is just syntactic sugar over rec.A.
PREVIOUS() is only special in that it automatically creates an index, which is important for efficiency. But generally you can look up a set of records and write multi-line Python code to do whatever you want with it and return a value. And it all runs in a sandbox with no access to anything except this document's data.
I reckon it makes more sense to return a table, and then have an index lookup in that table: e.g., index(PREVIOUS(blah_blah), column). Then you can also use an indirection to choose which column from another cell if desired.
This is how google finance in sheets returns some data (like exchange rates, iirc).
The root issue is that there's almost 50 years of spreadsheet syntax to consider and not colliding with.
The syntax strangeness continues:
Does that really convey most easily what is wanted? It seems needlessly verbose. Why would it require both $Person and $Date? Should that not be easier to figure out in its own right? Is that efficient?People need to remember that mentally, unless they use AI.