The magnetic fields generated by MRI (magnetic resonance imaging) machines and NMR (nuclear magnetic resonance) spectrometers are exceedingly strong. People have died when oxygen tanks, wrenches, fire extinguishers and such have accidentally been brought into a room housing such equipment. That's where these specially-designed and tested extinguishers come in. Each is crafted from non-magnetic materials and then tested and rated for application in the presence of magnetic fields.
Note: In the Cleanguard line, only the model FE13NM is non-magnetic!