All documents are associated with one or more objects e.g. applications, licences, persons, organisations etc. This is referred to as the document context. 

When a document is uploaded against an application, it may also be automatically associated with the licence and the licence holder. This is an attachment.  

A documents parent context is the primary object that the document is linked to. In the case of correspondence, this is driven by the template type.  i.e. a licence notice may be generated on the application, but the parent object is the licence, so the licence would be the parent context.  Parent context for an attachment will more often be the record it is uploaded against.