Putting an * beside all outstanding Invoices on a statement

You only need to exit the layout you use for your statements and add the variable AUDIT_HEADER.PAID_STATUS to where you want the indicator to go (eg just to the left of the invoice number - reduce the box down to a sensible width and right justify the contents) in the "HEADER_NUMBER footer" line and save it as the layout to use in future. Then when you create a statement to send to your customers a 'p' (part paid) or '*' (all outstanding) will be displayed against outstanding items, or nothing if fully matched.

If you would rather always have a star, then edit the expression of the added variable to say:

AUDIT_HEADER.PAID_STATUS="" ? "" : "*"

and a star will be shown for all items with an outstanding amount (not matched).

Note that this includes outstanding (unmatched) payments received. If you would rather only invoices were so marked, then adjust your expression viz:

AUDIT_HEADER.TYPE = "SI" ? AUDIT_HEADER.PAID_STATUS : ""

or

AUDIT_HEADER.TYPE = "SI" ? (AUDIT_HEADER.PAID_STATUS="" ? "" : "*") : ""

depending upon whether you want part paid (p) to be shown or just some/all outstanding (*).

You may want to edit any statement footer (ACCOUNT_REF footer) to include explanation of the markings you have added.