Standards will allow interoperability among stakeholders in the upcoming super-connected world. A smart manufacturing reference model (SMRM) is under development inside JWG21 between ISO and IEC. Based on a dimensionality analysis and the skeleton meta-model, the eight proposed SMRMs are reviewed and compared. The SMRMs are classified according to the number of lifecycle axes and the number of dimensional axes. Also, how the concept of a digital twin can be accommodated in an SMRM is investigated.