If the IEC attribute CONSTANT is set, the Access Level shall be read only.
The IEC standard does not define a key word to set the Access Level for a Ctrl Variable. The configuration in a programming system is vendor specific but it is recommended to provide a configuration option for the OPC UA Access Level.
When using the PLCopen XML format the AccessLevel shall be provided in PLCopen XML Additional Data in the XML element addData using the XML element UaAccessLevel as part of the XML element representing the variable. The value is a bit mask where the first bit indicates the read access and the second bit indicates the write access.