In R4, you could link your DocRef to any relevant context using DocRef.context.related.  However, in R5, that generic context linking went away, and the alternatives of subject and basedOn are limited in the resource types that can be referenced.

I need to do some homework to find all the resource types we currently reference from context.related today, but I think there are more than are allowed in subject or basedOn.  Also, the nature of the context isn't always best described by the semantics of subject or basedOn