Invoice numbers should run without interruption. A skipped number raises the suspicion that the document was issued and then removed from the records, and it becomes one of the first things to explain in an audit.
The Number Continuity report looks for gaps between the first and the last number of the year. It checks every candidate number a second time; if the document is recorded under a different date, it is shown separately as a back dated entry rather than as a gap. The two are different things and should not be confused.
On the e-document side the number is not free in any case: it is sixteen characters made up of a three character series, a four digit year and a nine digit sequence. The program locks the counter per series and per year, so even if two users save an invoice at the same moment, they never receive the same number.