The ion-thruster tag is used on 125 questions and it's usage guidance is:
Questions about engines used in space that accelerate ions to create thrust.
The electric-propulsion tag is on 59 questions, and it's usage guidance is:
Question: Is electric-propulsion a catch-all tag for several kinds of propulsion that may not overlap with ion-thruster? Perhaps things that use charged particles in the solar wind or existing magnetic fields along with electrical power in a spacecraft? Or should these two also be synonymized?