Designed from the ground up for use in conjunction with the West Tek power armor frame, the next generation underarmor is a form-fitting suit designed for maximum comfort and utility while operating the armor. Although it (or the hood) is not a requirement for operating the armor, it makes long-term use more comfortable and reduces chafing. The hard points and attachments on the suit also improve performance, interfacing directly with the armor's systems. The suit wasn't designed for use outside of armor: It features only two pockets on the hips for documentation and maps and a simple flap over the crotch. Its comfort and lack of bulk allow the operator to combine it with a variety of tactical gear and protection, for example, combat armor.

The Brotherhood employs pre-War underarmor not just in its intended purpose, for use by troops equipped with suits of power armor, but as a general purpose uniform. Typically combined with customized standard combat armor, the uniform is issued in three types: Olive drab for soldiers, orange and white for knights and jet black for officers of the Brotherhood. Non-standard customizations are popular: Lancers usually combine their uniform with a bomber jacket, while Elder Arthur Maxson wears his with an armored battlecoat