(courtesy of OP Innovations, Ltd.)
top of forehead: for sleep, meditation, head posture and activity, etc.
side of forehead: for alpha wave, left or right brain activity, etc.
center of chest: HRV, breathing, body posture and activity, etc.
slant over temple: cool Star-Trek 7-of-9 look, alpha wave, eye movement, etc.
behind ear: cool sci-fi star crew look.
both sides of forehead: double cool, left & right brain compare.
upper arm: body activity, SCR/GSR/EDA, etc.
wrist, top side: hand activity, SCR/GSR/EDA, etc.
wrist, bottom side: hand activity, SCR/GSR/EDA, etc.