The messaging implementation manual (MIM) defines HL7v3 message standards. Individual interfaces within this interoperability standard will refer to specific releases of the MIM where their messaging specifications are defined.
The official source of MIM documentation is the Terminology Reference Data Update Distribution (TRUD) MIM release page.
A quicker source for reference is the online MIM pages on the NHS API Hub under the section Spine APIs.
The latest version of the MIM is 7.2.02. This version will apply unless otherwise stated.
Note some interfaces within the standard do use earlier versions of the MIM.