Part 7 describes support for the use of IndexRange when the Attribute value is an array. For a string array, a client can access the nth element of the array and the server would return only the nth element of the array in a read response or publish response.

Test case 024.js in "Monitored Item Services\Monitor Value Change" is one example where IndexRange is used for a scalar string, in order to attempt to access a substring of a scalar string.

Part 3 defines the string data type as a unicode character string, not as an array of characters

Following Part 7 and Part 3 of the specification, it cannot be expected that an OPC UA server returns a substring of a scalar (non-array) string when the IndexRange is used. (The same that it cannot be expected to return a bit at a certain position within a byte, when the IndexRange is used with a byte type). Also there is no mechanism to use the IndexRange to access a substring of a string element of a string array.