HL7 2.8 is the first that comes to mind, and it is the most relevant one. HL7 2.x versions include 2.1, 2.2, 2.3, 2.3.1, 2.4, 2.5, 2.5.1, 2.6, 2.7, 2.7.1, 2.8, 2.8.1 and 2.8.2. The v2.x standards are backward compatible. HL7 2.x earlier versions are currently the most widely used, but middleware solutions should support newer versions 2.7.1, 2.8, 2.8.1, and 2.8.2.