PAY: Help with column source_action_id in table pay_assignment_actions

 I've searched all over and cannot find the true meaning of column source_action_id in table pay_assignment_actions. More to the point, I've inherited code that runs a relatively simple query to retrieve all specific payroll dates from pay_payroll_actions based on effective_date being between a date range. These dates are then used to pull 401k balances for the effective date. In this query, pay_payroll_actions is joined onto pay_assignment_actions solely to verify that for a given payroll, source_action_id is NOT null. If it is null, the query will not consider it for the 401k query.