Whenever a custom object record is referred to in other records or appears, for example, in search screens, it appears as a hyperlink. This hyperlink appears in one of the four available formats:

Project Link Display Options

You may select the desired format on the Unique ID tab of the custom object for the automatic and manual numbering options. Existing records keep the previous settings only until a user makes changes or updates then. When a user updates an existing record, the new setting you specified takes effect.