This document describes: the rationale for building customisation supporting business cases that are specific to their business environment while maintaining organisational and semantic interoperability with the TC 440 specifications; the difference between Usage specification and Extension specification; a methodology on how to define customisations on: BII Transaction specification, Business rules, Code lists; how to claim compliance or conformance to a customisation of a TC 440 specification; the connection to the eProcurement Ontology project. This specification does not describe the detailed process of building an extension.