One AR invoice for Multiple Sales Orders

Hi,

We are on R12.2.4

We have a scenario where there is one AR invoice for multiple sales orders. (Ofcourse, Bill to, Ship to, Currency, SO ship date etc... are same)

I can see Sales order at AR invoice header (under reference field) as "XXXX". This invoice has three lines and for each of the lines, Sales order tab shows Sales order number as "XXXX", "YYYY" and "ZZZZ".

I have two questions:-

1. I know system is creating one such AR invoice (based on AR Grouping rules). But on what basis is the system updating the reference number field (under AR transaction header) with one of such Sales Orders.