Hello.
This is very good question.

You can use SubscribeMultipleItems and SubscribeItem calls interchangeably. And, functionally, the end result (how the subscriptions work once they are established) will be equivalent, no matter which combination you use to establish the subscriptions. Same applies to unsubscribing.

But, there will be performance differences with regard to the time needed to perform the "subscribe" or "unsubscribe". When you make more method calls (on the QuickOPC API level), you end up with more function calls on OPC COM/DCOM level. And each such call has its own latency/processing time. But the OPC functions are designed in such a way that multiple subscriptions/unsubscriptions can be performed using one call. So, making 100 calls with 1 item each is going to take longer than 1 call with 100 items in it. For this reason, if the items you want to subscribe to (unsubscribe from) are available to you at once (and do not come in, time-wise, one by one), it is recommended that you use SubscribeMultipleItems, with large/larger number of items in it, and not individual SubscribeItem calls.

Note: SubscribeItem is just a short wrapper around SubscribeMultipleItems which passes in an array of one element, for ease of use. What matters is the number of items in the call, not the method name.

Best regards