If a document has the status Approved, it cannot be edited any more because it was published. Also, the document of another staff member cannot simply be changed. If a document is to be changed nevertheless, a new version that has the In development status is required.
When copying documents, the system initially assumes that a new version of the source document should be created, and therefore asks "Create new version?". Yes causes a new version to be provided. The Number of the new document stays the same, and its Version increases by 1. If you respond No, then a new document with a new document number is created, and both the version number and the revision number are set to 0.
Document numbers together with version and revision numbers make the document unique. The combination of these three values is always checked for uniqueness.