How are receivables transactions automatically numbered if document sequencing rules are NOT set up?

Summary:

When creating an invoice from a "manual" source the transactions are automatically numbered which is in agreement with the setup task "Transaction Sources" where the setting for Manual source is Automatic Transaction Numbering = TRUE.

However, there are NOT any document sequencing rules configured in the environment.

How are invoices automatically numbered in this case? Additionally, receipt numbers are manually entered, why is this the case?

Content (required):

Version (include the version you are using, if applicable):

23A (11.13.23.01.0)

Code Snippet (add any code snippets that support your topic, if applicable):