The Node ID refers to the node, or some attribute of the node, as a whole. If the data type of that attribute is an array, then by specifying just the node ID you are referring to the array as a whole, and when reading or subscribing, you will be receiving values that contain the whole array. There is no (server-independent) way to specify an array element or a range of elements inside the node Id.
But that is not the whole story. When reading or subscribing (or writing), you can specify additional parameters besides the Node Id. What you need to use is "index range list". Following is an excerpt from our Concepts doc that covers that:
4.13.5 Index Range Lists
OPC Unified Architecture has support for single- and multidimensional arrays in data values. In addition, it is also possible to work with (read, write, subscribe to) only a subset of an array, so that the whole (potentially large) array does not have to be transferred between the OPC server and OPC client.
QuickOPC-UA uses Index Range Lists (UAIndexRangeList objects) to control which parts of array value should be accessed. An index range list can be used to identify the whole array, a single element of a structure or an array, or a single range of indexes for arrays. Index Range List is basically a list of individual Index Ranges (UAIndexRange objects), each containing a range for a single dimension of an array.
There is an implicit conversion from an integer (Int32) to a UAIndexRange, so if your language (such as C#) supports implicit conversions, an index range containing just a single element can be written simply as the element index, without explicitly constructing the UAIndexRange object.
Besides constructing the UAIndexRangeList from individual UAIndexRange objects, it is also possible to specify the index range list by parsing a string, using static UAIndexRangeList.TryParse method.
Examples of index range strings:
5:20
10:11,1:2
1,4:7,90
Dimensions are separated by commas. A minimum and maximum value, separated by a colon, denotes a range of index. If the colon is missing, only a single for an index is selected. An empty string denotes the whole array.
One-dimensional index range lists can be easily constructed using the UAIndexRangeList.OneDimension static method, passing it either the single index, or minimum and maximum indices.
UAIndexRangeList can be given as IndexRangeList property of the UAAttributeArguments object (and derived objects, such as UAReadArguments, UAWriteArguments, UAWriteValueArguments, EasyUAMonitoredItemArguments, and EasyUADataChangeArguments<>). You can then pass this object to a corresponding method on the main EasyUAClient object.
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