A study arm is "a group or subgroup of participants in a clinical trial that receives specific interventions, or no intervention, according to the study protocol. This is decided before the trial begins" (ClinicalTrials.gov). "Describes an expected sequence of events for one of the participants of a study. E.g. Exposure to drug A, wash-out, exposure to drug B, wash-out, follow-up." - HL7 FHIR, ResearchStudy.arm