Siemens S5-135U/155U System Manual page 29

Depending on the propagation medium (conducted or non-conducted
interference) and distance from the source, interference can be picked up by
the programmable controller via different coupling mechanisms.
A distinction is made between the following:
System Manual
C79000-G8576-C199-06
Direct coupling
Capacitive coupling
Inductive coupling
Radiated interference
Installation Guidelines
3-3