Self Powered. An instrument that uses an internal power supply to provide 24VDC loop power. Often found when the instrument requires higher power or additional electronics. Instantly recognisable as it needs 4 wires. (Two for power supply and two for 4-20mA signal

Loop Powered. An instrument that requires an external 24VDC power supply. (Instantly recognisable as it only requires 2 wires (Two for 24VDC over which the 4-20mA is imposed)

System Powered. An instrument that "expects" that the I/O module provides the 24VDC power. (In reality it has no idea if the 24VDC is from the I/O module or a loop power supply)

感觉这是二线制和四线制的标准解释