The intention of the OPC UA standard is not to define a set of units but a way to expose units based on existing systems. Since there is not a single worldwide set of units used in all industries, the EUInformation structure includes a separate field (the namespaceUri) to identify the system on which the exposed unit is based.
The default OPC UA mapping is based on UN/CEFACT as defined in clause 220.127.116.11, because it can be programmatically interpreted by generic OPC UA Clients. However, the EUInformation structure has been defined such that other standards bodies can incorporate their engineering unit definitions into OPC UA. If Servers use such an approach then they shall identify this standards body by using a proper URI in EUInformation.namespaceUri.