Are you referring to heat acclimatization for unit life or BMT?
In BMT, I recall my PC/3SG would instruct the platoon to wear our LBV during our breaks in between for heat acclimatization which was frankly useless because we all be in our admin attire wearing it and with our bunk fans turned on.
In my unit, our encik would never allow any of us to wear admin attire underneath our long 4 once we go outfield because it would increase the chance of us getting heatstroke. Made sense after all who would want to wear an extra layer of clothes underneath the long 4
No idea whether the above applies to the rest but this was what happened to me during my NSF days.