The numbering logic is complex. It keys off of the Invoice Preference setting for the invoice which in the preferences table, the record for that value specifies the Invoice Numbering Group to be used. This then ties back to records in the si_index table keyed by the constant node value of “invoice” and the sub_node value that comes from the Invoice Number Group. If you are confused, I don’t blame you.
To help, I uploaded changes for 2019.2.8 version that adds information to several screens and I updated the “How To …” topic for the “change the invoice number starting point” topic accessed from the menu on the left.
You need to have things set up correctly to get numbering to work. If you want any type of invoice to use that same numbering sequence, in the Inv Prefs records (under the Settings tab) should have the Invoice numbering group set to Invoice. Then they all use the same record.