The AccessPath is intended as a way for the client to provide to the server a suggested data path (e.g. a particular modem or network interface). It indicates HOW to get the data.
The ITEM ID provides all of the information needed to locate and process a data item. The Access Path is an optional piece of information that can be provided by the client. Its use is highly server specific but it is intended to allow the client to provide a `recommendation' to the server regarding how to get to the data. As an analogy, if the ItemID represented a phone number, the access path might represent a request to route the call via satellite (or transatlantic cable or microwave link). The call will go through regardless of whether you specify an access path and also whether or not the server is able to use that suggested path.
For example, suppose you wanted to access a value in an RTU and had a high speed modem on COM1 and a low speed modem on COM2. You might specify COM1 as the preferred access path. Either one will work, but you would prefer to use COM1 if it is available for better performance.
In any case, the use of access path by both the server and the client is optional. Servers need not provide the function and clients need not use it even if it is provided.
Servers which do not support access paths will completely ignore any passed access path (and will not treat this as an error by the client). Also, when queried, such servers will always return a null access path for all items (i.e. a NUL string).