Syntax References represent established works of other organizations. Such works – among others – can define semantic information, topologies, a language for a code set, or dictionaries and facilitate the programmatic evaluation or the lookup of a quantity or unit e.g. in a dictionary or ontology. They can be publicly defined by standard bodies such as IEC or proprietary (e.g. vendor-specific dictionaries).

The Quantities and Unit Model provides a powerful way to relate quantities and units to such Syntax References using the Dictionary References model defined in OPC 10000-19.

Table 33 lists Syntax References that are applied in certain markets. This list can be extended in future versions. Vendors or organizations can also specify additional Syntax References.

An overview for each Syntax Reference in this table is provided in Annex C.

Table 33 – List of Syntax References

Common Name	Syntax Reference URI	Full Name
UCUM	https://ucum.org	Unified Code for Units of Measure
QUDT	https://qudt.org	Quantities, Units, Dimensions and Types Ontology
IEC CDD	https://cdd.iec.ch/	Common Data Dictionary
UNECE	https://unece.org/trade/uncefact/cl-recommendations	Codes for Units of Measure Used in International Trade
LATEX_SIUNITX	https://www.namsu.de/Extra/pakete/Siunitx.html	LaTeX SI Unit Extension