The content of an Instance is a reflection of the content in the resulting file(s) on disk.
An instance may contain both content and metacontent, added via .append() and .data[] respectively. You can think of the contents of an instance as the bytes for a file, whereas metacontent represents additional information about an instance, such as author and date modified.
When used within a host, the content typically refers to the part of a workspace that is to be published, such as a the physical nodes making up a character model or rig.