Any number's template can be used for multiple document types (templates). Each document type (template) has its own unique by default.

If you want to "pass-through" the numbering of documents without a document type, you should disable the unique by checkbox.

Default	Disable
Invoice 001 Contract 001 Invoice 002 Invoice 003 Contract 002 Invoice 004 Contract 003 etc.	Invoice 001 Contract 002 Invoice 003 Invoice 004 Contract 005 Invoice 006 Contract 007 etc.