In the general order/request space, it's common to order multiple things at once and link them with a 'grouping' id which may influence how the requests are processed and how results are reported (e.g. whether tests can share a common specimen draw, whether results are reported together, how activities are billed for, etc.), however the individual requests can still have their statuses manipulated independently.  The general order/request space also has a RequestGroup resource that can be used when multiple things are ordered simultaneously but there are dependencies between different Requests that prevent their states from changing independently. In the claims space, the claim 'items' can be processed/evaluated at different times, but the overall evaluation is done in the context of the complete set of items.  It is not possible for either a claim or a pre-determination to have items 'removed' or 'added' post-submission without cancelling the entire claim/pre-determination.  As such, the Claim always acts as though all items are organized into a RequestGroup.  The same rules generally apply to prior authorization - with one exception - the U.S.  X12 allows (and a small subset of payers support) the addition and removal of items from a Prior Authorization request without cancelling and then re-submitting the entire prior authorization.  In this one aspect, Claim items do behave somewhat like individual independent requests with a grouping id However, because this behavior is such an exception to the norm, it does not make sense to try to model ClaimItem in the same manner as other types of FHIR requests.  Doing so would impose considerable complexity on the very typical case in order to create alignment for an exceptional case.