I had to read the existing wording several times and also refer to the existing R4 Subscriptions page to understand it. I had particular difficulty with understanding the phrase "defined by a client at run-time via a query". This makes me think that the client somehow defines the subscription by querying the server.

Existing Wording:

In those releases of FHIR, subscriptions are defined by a client at run-time via a query. The FHIR server must then run the query and track the query result-set for each subscription request.

Proposed Wording:

In those releases of FHIR, subscriptions are defined by a client dynamically by posting a Subscription resource with a criteria string. The FHIR server must then run a query against that criteria and track the query result-set for each subscription request.

(Comment 31 - imported by: Gino Canessa)