On some points (currently, on all point available: OPC-DA Item Point, OPC-DA Property Point, and OPC-UA Attribute Point), it is possible to use client-side array element extraction.

For array values, this allows you to work with just one element at the given position. With each point, you can specify an index or a list of indices in the ExtractArrayElementIndexes property, and for reads and subscriptions, OPC Data Client will extract the specified element.

Here is how the client-side element extraction is configured in the property grid (the Properties tab of the Point Editor).

In the "Extract Array Element Indices" row, you can either manually enter an index or a sequence of indices separated by commas, or you can press the "..." button on the right side to invoke a Collection Editor for the indices.

The client-side array element extraction is mainly useful with OPC Data Access "Classic" servers, because OPC-DA has no corresponding feature in the specification. In OPC Unified Architecture, on the other hand, it is possible to specify Index Range List with each read, subscribe, or write. If supported, using the Index Range List in OPC UA is recommended over the client-side array element extraction. But not all OPC UA servers support this feature, and in such case you may want to extract the array elements on the client side.

The number of specified indices must match the rank (number of dimensions) of the array, and you can only extract one element (not a range of elements), otherwise an error occurs. 

Reads and Subscriptions

For reads and subscriptions, OPC Data Client will simply read or subscribe to the whole array, and extract the specified element.

Writes

For writes, OPC Data Client will first read the whole array, then modify the element, and then write the whole array back. This read/modify/write algorithm is potentially dangerous if the values are changing in the server or from other clients, and can be disabled using the AllowReadModifyWrite property (defaults to true). When this is disabled and client-side element extraction is specified, the point becomes read-only.