OBJECTIVE: The Outcome Measures in Rheumatology (OMERACT) Filter 2.0 framework was developed in 2014 to aid core outcome set development by describing the full universe of "measurable aspects of health conditions" from which core domains can be selected. This paper provides elaborations and updated concepts (OMERACT Filter 2.1). METHODS: At OMERACT 2018, we discussed challenges in the framework application caused by unclear or ambiguous wording and terms and incompletely developed concepts. RESULTS: The updated OMERACT Filter 2.1 framework makes benefits and harms explicit, clarifies concepts, and improves naming of various terms. CONCLUSION: We expect that the Filter 2.1 framework will improve the process of core set development.