Content is large chunk of data that is generated within a component.
One example of content is a PDF file generated by the reporting component. You cannot specify a default value for content since it can be of any type and is represented internally as a byte stream.
This XML defines a content node named attachment that is expected to exist in the runtime context under the name report-output. In this example, the report component generated a document and stored it as report-output. The email component could embed this data as an attachment in an email.