ISO 81346-12:2018 This document establishes rules for structuring of systems and the formulation of reference designations and provides classes for systems in the field of construction works and building services. This document also specifies a classification of objects and corresponding letter codes for use in reference designations of object occurences. This document is not intended for manufacturers or system-related designations of individuals (e.g. inventory number or serial number) or for product types (e.g. article number or parts number).