Both OPC subscription, and OPC asynchronous reads/writes require a callback from the OPC server to the OPC client. In this sense, they are quite similar, and a subscription can be considier a special case of the asynchronous OPC communication.
The asynchronous read/writes, however, are "one-shot" transactions, initiated individually by the OPC client. The subscription is created inside the OPC server by the OPC client just once; since that moment, the OPC server itself takes care of communicating with the device, and reporting changes to the OPC client. The OPC server does this until the OPC client explicitly unsubscribes.