SCADA stands for Supervisory Control and Data Acquisition. Although they are sometimes used interchangeably in casual conversations, some components are more common for one than the other.
An HMI is usually a stand-alone terminal that can communicate with one (most commonly) or more PLCs in the field. Such terminals may be distributed throughout the plant and are often unaware of each other.
A SCADA system will be connected to a large number of control systems. It will access PLCs and HMIs throughout the plant via the network. based on the information collected, the SCADA system will process the information and deploy it to the appropriate terminal or HMI. it knows all the HMIs in the field and will intelligently assign the information to be displayed.