It is straight Python. Yes the spreadsheet habit of making functions SHOUT does look weird mixed in with Python syntax.
PREVIOUS returns a record, then .Running_Balance pulls out one field of it.
PREVIOUS returns a record, then .Running_Balance pulls out one field of it.
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).
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.