Request.instantiatesCanonical *has the following comment, which seems misaligned (better aligned with *Request.identifier)

Note: This is a business identifer, not a resource identifier (see discussion). It is best practice for the identifier to only appear on a single resource instance, however business practices may occasionally dictate that multiple resource instances with the same identifier can exist - possibly even with different resource types. For example, multiple Patient and a Person resource instance might share the same social insurance number.