SUM adds all the values you provide in between a set of brackets, each seperated by a comma, and returns the result.
SUM can also return the added total of values that are produced by another function.
SUM(p.base_fee, p_transaction_fee, p.report_fee)
This expression works in a couple of steps to bring you the result:
SUM( .... ) is the overarching function, of which you want the result. In between its brackets sits another function.
get_properties_from_instances(.......) is the function inside the
SUM brackets, which goes and fetches the property of the instances you specify.
instances(AccountHolder, personal_income) returns the personal income of all the accountholders.
These three combined, give you the sum total of the personal incomes for all the accountholders in the case.