The IODD data type StringT can either use ASCII or UTF-8 encoding. It is always mapped to an OPC UA DataType String, which is UTF-8 encoded.
The mapping requires that IO-Link ASCII based strings are converted to UTF-8. Mapping an ASCII string to UTF-8 can always be done but mapping an UTF-8 string to ASCII may fail (when writing a UTF-8 string to an IO-Link Device expecting only ASCII strings). In case the OPC UA Server cannot do a transformation, the operation shall fail.
If the value can directly be mapped to a Variable (see section 12.3), the BaseDataVariableType is used, and the mandatory element fixedLength is mapped to the Property MaxStringLength defined in OPC 10000-3, and the element encoding to the Property Encoding (see section 13). If the Variable is referenced by a StdVariableRef, and the fixedLengthRestriction is defined, this needs to be used instead.