The Tri-field Natural EM Meter was designed to detect extremely weak DC fields and ignore AC fields. This is why it is believed to be a good measurement tool in paranormal research based on the theory that a spirit retains its energy after death.
The "magnetic" setting detects movement of magnetic field sources at a location. The meter will sound when a magnetic field increases or decreases. When the field becomes stable for at least 5 seconds, the meter will return to a base reading.
The "electric" setting causes the meter to become sensitive to electric fields as weak as 3 volts per meter. This is EXTREMELY sensitive and has been designed to ignore "background noise." The setting is so sensitive that it can pick up the electric fields of humans and animals - even through walls.
The "sum" setting combines the readings of the previous two settings. This setting is so sensitive it can detect a field change up to 5-10 feet away. However, the meter will not differentiate between the two fields (magnetic and electric).
It is recommended in Ghost Hunter’s Guidebook by Troy Taylor, to use the magnetic setting during indoor investigations. The setting will allow the detection of magnetic field fluctuation without the possibility of AC fields contaminating the readings. Because of the meter’s sensitivity, if the electric or sum settings are used indoors, all investigators should remain very still and at least 5-10 feet away from the meter. In these settings, the meter is quite capable of reading an entire room.
EDITED TO ADD:
After more thought and a little more reading I would like to add a thought. If using the "sum" or "electric" reading, I feel it’s important to not have any other electric device in close proximity to the meter. Although the meter is designed to ignore AC currents, it is designed to read DC currents. As researchers, we use a wide array of equipment with DC currents: flashlights, audio recorders, thermometers, etc. If the meter is capable of picking up the electric currents in the body of a human, it should easily be able to read the DC currents in any of the equipment we use batteries in. This is just something to think about when using the "electric" setting in any form and trying to collect accurate data.