The electrical circuit will always take the path of least resistance so placing electrode on either side of an area is best to try and make sure the current flows where you want it; and avoid where you don't want it. Its important with both to make sure you have a good connection to your skin.
Examples. Placing slippers on each foot will mean that your legs are receiving frequencies but it is unlikely that your arms would receive much (if anything). Placing on right hand and left foot will mean that there is more chance that frequencies will travel through your liver as they go from a to b.
Use common sense. It's recommend to keep contact electrodes at shoulder level or lower to avoid brain and similarly do not target the heart area.
There isn't really a difference to using tens or cylinders in terms of location apart from convenience of holding them in place.
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