The SCPI Standard specifies a command structure and syntax for programmable instruments control. The physical communications link, such as GPIB (IEEE-488) , RS232, USB, VXIbus etc, is NOT defined by SCPI. SCPI also includes standard command sets for several classes of instruments, e.g. power supplies, loads, and measurement devices such as voltmeters and oscilloscopes. A predecessor of SCPI, but with more limited scope, was IEEE 488.2 in 1987.
SCPI commands are ASCII textual strings, which are sent to the instrument over the physical layer, such as GPIB.
Two examples: With the command SYSTem:COMMunicate:SERial:BAUD 2400 the baudrate of an RS232 interface can be set to 2400 bit/s. With the command SYSTem:COMMunicate:SERial:BAUD? the current baudrate of an RS232 interface can be queried. Commands can be abbreviated to their upper case parts, for instance SYST:COMM:SER:BAUD?.