Quick Answer
A sub-resource is a resource, which means it can have properties, sub-properties, and be as complex as any resource. The critical difference is that sub-resource do not appear alone but rather as members of a parent property. This is similar to how sub-properties are displayed, but there can be more than one sub-resource belonging to a parent property. For this reason, sub-resources are contained within a JSON array.
Digging Deeper
There are practical and academic answers to this question. Both are important.
