Make text more clear. Suggestion: "Each event in the Event Catalog defines what context is expected in the notification. The context contains zero, one or more FHIR resources. Hubs MAY use the FHIR _elements parameter to limit the size of these resources data being passed by specifying the elements to be included in the in message (e.g. the identifiers of a resource). When they do so they SHALL include all elements indicated in the event definition.
Subscribers SHALL accept a full FHIR resource or the _elements-limited resource as defined in the Event Catalog."

Existing Wording:

Each event in the Event Catalog defines what context is expected in the notification. Hubs MAY use the FHIR _elements parameter to limit the size of the data being passed while also including additional, local identifiers that are likely already in use in production implementations. Subscribers SHALL accept a full FHIR resource or the _elements-limited resource as defined in the Event Catalog.

Proposed Wording:

Each event in the Event Catalog defines what context is expected in the notification. The context contains zero, one or more FHIR resources. Hubs MAY use the FHIR _elements parameter to limit the size of these resources data being passed by specifying the elements to be included in the in message (e.g. the identifiers of a resource). When they do so they SHALL include all elements indicated in the event definition.
Subscribers SHALL accept a full FHIR resource or the _elements-limited resource as defined in the Event Catalog.