See FHIR Connectathon 16 thread

https://chat.fhir.org/#narrow/stream/implementers/topic/XDS.2FMHD.20.2B.20Transaction.20Semantics

on the FHIR spec at

http://build.fhir.org/http.html#2.21.0.17.2

current text says:

A transaction may include references from one resource to another in the bundle, including circular references where resources refer to each other. If the server assigns a new id to any resource in the bundle as part of the processing rules above, it SHALL also update any references to that resource in the same bundle as they are processed. References to resources that are not part of the bundle are left untouched. Version-specific references should remain as version-specific references after the references have been updated. Servers SHALL replace all matching links in the bundle, whether they are found in the resource ids, resource references, url elements, or <a href="" & <img src="" in the narrative.

I propose that this same processing rules be clearly applied to URL datatype, not just Reference.

For example the IHE MHD profile needs this.