Counter intuitively, it refers to a type of covert upper armor clothing. These are T-shirts and undershirts that have bulletproof panels installed in them.
While open military conflict requires hard plates and breathable innerwear, the best option for domestic and urban conflicts are covert T-shirts due to the weight and protection area.
US Armed Forces do not have standardized underwear. Generally, it is recommended for soldiers to wear comfortable and breathable undergarments with at least 50% cotton.