The ability to add attachments to an object is the ability to upload and associate a file, link to a url, or describe an external location of content and associate that content data with this object. For example you may create a document and upload one or more files. You may also add multiple urls that point to additional information as supporting material for a change request.

Object types such as documents implement an interface called FormatContentHolder which allows them to associate one piece of content as “primary content”. There can only be one primary content though. Object types such as a change requests implement ContentHolder which allow them to have multiple secondary content but not primary attachments. Objects that implement FormatContentHolder however can have both primary and secondary attachments because FormatContentHolder is a specialization of the ContentHolder interface.