Parameterized ValueSets are a mechanism (documented in the FHIR Tools IG) for value sets whose expansion depends on a runtime parameter — choose country, then the state list narrows. The pattern is built from extensions (valueset-parameter, CQF expression on filter values, binding-parameter) and is evaluated under Structured Data Capture (SDC) rules in Questionnaires.

This is the machinery behind dynamic DTR questionnaires: answer A changes the options for question B. Payer forms often need diagnosis→body-site or plan→covered-service pick lists that cannot be a static code list baked into the Questionnaire.

What ImOnFHIR does with it: we treat this page as the placeholder spec for the future DTR Forms Lab. Our terminology handling must anticipate $expand calls that carry p-* parameters. We point at HL7's open docs; we do not host licensed code-system content (CPT, etc.) as answer lists.