SCADA shows what the process is doing. MES adds production context such as orders, batches, quality, genealogy, traceability and execution.
In a typical ISA-95 architecture, SCADA primarily sits at Level Two (2), while MES primarily operates at Level Three (3).
HMI, alarms, machine states, trends, process variables and supervisory control.
Orders, batches, production, quality, genealogy, traceability and electronic production records.
PLC → OPC UA or industrial server → SCADA/Edge → MES → ERP. MES does not need to directly control every PLC in order to understand production.
SCADA may know that a pump is running. MES can know which order is being executed, which material is being used and whether the finished product is traceable.