Most SCADA packages work like a HMI as a way to view and enter data into a PLC. The difference between a SCADA system and a HMI is generally in size, scale, and scope. HMI's such as Panelviews are usually simpler and machine specific. SCADA on the otherhand usually more plant specific and controls many machines or a process of multiple machines. Generally it scada does much more complex trending and provides much more information, but as far as how it interacts with the PLC, it is the same as an other HMI, just more powerful.