This clause specifies how to apply the “Codes for Units of Measurement” published by the “United Nations Centre for Trade Facilitation and Electronic Business” (see https://www.unece.org/cefact/codesfortrade/codes_index.html). This recommendation establishes a single list of code elements to represent units of the International System of Units (SI Units) like units of measure for length, mass (weight), volume and other quantities and in addition covers administration, commerce, transport, science, technology, industry etc. It provides a fixed code that can be used for automated evaluation.

Table 21 contains a small excerpt of the relevant columns in the UNECE recommendation:

Table 21 – Examples from the UNECE Recommendation

Excerpt from Recommendation N°. 20, Annex 1
Common Code	Name	Symbol
C81	radian	rad
C25	milliradian	mrad
MMT	millimetre	mm
HMT	hectometre	hm
KMT	kilometre	km
KMQ	kilogram per cubic metre	kg/m3
FAH	degree Fahrenheit	°F

The UNECE recommendation in several cases defines multiple instances of the same unit (same name and symbol) for different quantities. Therefore, the relevant information for EUInformation.unitId, EUInformation.displayName, and EUInformation.description has been extracted by eliminating duplicates. This extract is available here:

http://www.opcfoundation.org/UA/EngineeringUnits/UNECE/UNECE_to_OPCUA.csv

This mapping has been generated as follows:

• The namespaceUri shall be http://www.opcfoundation.org/UA/units/un/cefact
• The Common Code (represented as an alphanumeric variable length of up to 3 characters) has been converted into a 32 Bit Integer and is used for the unitId. The following pseudo code specifies the conversion algorithm:

Int32 unitId = 0; Int32 c; for (i=0; i<=3;i++) {

c = CommonCode[i]; if (c == 0) break; // end of Common Code unitId = unitId << 8; unitId = unitId | c;}

• The Symbol field shall be used as invariant locale for displayName. Servers may configure multiple additional locales for each displayName. However, if none of the LocaleIds specified by the Client for the Session matches these additional locales, the Server shall return the invariant locale.
• The Name field shall be used as invariant locale for description. Servers may configure multiple additional locales for each description. However, if none of the LocaleIds specified by the Client for the Session matches these additional locales, the Server shall return the invariant locale.