A few issues: A single Composition can, in theory, be used as the root of many documents, each having a different subset of resources included in the Bundle (i.e. each with a distinct GraphDefinition) The semantics of "instantiatesCanonical" are about an event adhering to an order set, protocol, etc.  Not a description of what resources should be (or are?) inside a DocumentBundle.  If there's a desire to define a Composition that says "this is the set of resources that should be included in the Bundle when a document is generated with me as the root", that should be a different extension.  If there's a need to indicate that a specific DocumentBundle adheres to a specific GraphDefinition, that should be handled via something on Bundle.meta.  (We've talked about the possibility of Bundle.meta.profile being able to be GraphDefinition in addition to StructureDefinition, though so far, no action has been taken on this.) In either case, this isn't something we can back-port into R4.  So for now, a custom extension will be needed or you could possibly pre-adopt an R5-defined extension (once one is created).