It is a common requirement in Companion Specifications to define discrete values which have a string representation. A typical approach (independent of OPC UA) is to use enumeration data types, defining specific strings for internally used numeric values. The drawback on standardizing such Enumeration DataTypes in OPC UA is, that they are restricted exactly to those values. It is not allowed for a vendor to extend and even not in a new version of the specification (see 3.2.4).

There are different workarounds for this issue.