When creating a custom type, you get the option to add a new property of any type of database (ie page, weblink, etc) or content (text, number, etc).
however, sometimes it could be desirable to have a poperty that can link to
any
item in any or some selected databases.
For example, adding a "source" property to a note should allow to choose a weblink, audio, image, or custom type. and not limited to one type only.