VendSettlement - how does it work? Part 1

If I have a settlement of the invoice and a payment, in the vendSettlement I would have one record. If I then Undo the settlement it , I get in the vendSettlement table a second record that is a copy of the previous but with a negative amount

In the case that I look into there is something different:
One vendSettlement has
TransRecId = X OffsetTransId = Y and Amount=Z
and a second record has
TransRecId = Y OffsetTransId = X and Amount=-Z
What would the user do to achieve such result?
Thank you!