This is the ValueSet that contains all the codes in Modality that created the series (http://fhir.ch/ig/ch-rad-order/ValueSet/ch-rad-order-acquisition-modality) and Modality that created the series (http://fhir.ch/ig/ch-rad-order/ValueSet/ch-rad-order-acquisition-modality). E.g. what you have to deal with if you get resources containing codes in either of them