Documents can either be numbered manually or by using an autonumber mask. The option of whether to allow manual numberring, autonumbering, or both is configured at the Site level.
To turn manual document numbering on or off, a user with appropriate access may navigate to System Setup | Sys Configuration | Site | <Choose Site> | Autonumber | Edit. If you select "Optional" or "Never" for a document type (or modification), that will allow the option for "Manual Entry" to be available when creating a new document. If you select "Mandatory" or "Auto", the "Manual Entry" option for numbering a document will not appear.
Please note that all document numbers of the same document type must be unique; duplicate numbers can not be used, as this will result in an error in the system.