The Instrument Definition for the Muon instruments has evolved from that proposed in 2001 to the more robust definition outlined in the linked discussion document (pdfs/Muon_V2R7.pdf).
For completeness both are discussed in this document; however, the NIAC are asked to vote on and ratify the ‘Muon Instrument Definition: Version 2 - pulsedTD’
Aspects of the Muon Definition either introduce new NeXus Classes or extend existing definitions. These are discussed separately in the linked document (pdfs/Elements_for_Muons.pdf), and the NIAC are asked to separately consider and ratify these elements that are key to the Muon definition.