Have you ever seen a deleted document in the Posted Document, e.g. Posted Purchase Receipt?
That is because of this:
When you post the Purchase Order, at some point it will get the Purchase Receipt No. from the Receiving No. Series. It will increment from the Last Used No. in that Receiving No. Series. Once got, it will update the Last Used No in that No. Series. That no. will be used in the Receiving No. in Purchase Header.
At some point later, there is a COMMIT code, so that Receipt No. is already reserved. Once there is a COMMIT code and you get an error after the COMMIT (shown as red error on flowchart below), you can’t rollback to the initial state before the Purchase Receipt get the number. Hence the Receipt No. is already reserved.
Because of the audit trail requirement, Posted Document No. cannot be missing. But in this case, the Purchase Order has reserved the Receipt No. Hence, when somehow you must delete the Purchase Order, NAV will create (post) a Posted Document using that reserved number. But, with the line description as Deleted Document, to make it clear to the user, that this is created from a deleted document, instead of posting transaction.