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I need to build a catalog of items with billing codes provided by third-parties. I've looked at CatalogEntry and ChargeItemDefinition. 

CatalogEntry doesn't let me attach a price, so I've ruled it out for now
ChargeItemDefinition could work - but I can't set instance to ActivityDefinition for use cases like diagnostics.

What's the best way to model this?

ChargeItemDefinition is the right place.  Two potential paths:
1. Put the ActivityDefinition in an extension
2 Put the ActivityDefinition as a reference in the instance field (this is in accordance with R5 but is not exactly spec compliant for R4)

<@756044735814172743> building on <@800502084980965417> 's answer: 
I agree that `ChargeItemDefinition` is the right resource, but it is underfined. R5 made a lot of improvements to the billing / financial management resources, but honestly I wil say there is more work to be done

One additional resource I can point you to: 

The Order Catalog Implementation Guide (http://hl7.org/fhir/uv/order-catalog/2020Sep/index.html) defines a BillingCode and BillingSummary extension on the ActivityDefinition to solve this problem: http://hl7.org/fhir/uv/order-catalog/2020Sep/StructureDefinition-LabServiceDefinition.html