Army bags are specially designed to balance carrying weight and allow for easy transport of personal or company essentials. Also, it needs to be exceptionally durable and lightweight on its own.
The US Armed Forces generally use the MOLLE 2 rucksack, named after the modular webbing placed on all multiple sides of the pack to allow pre-mission adjustments.
Military backpacks should be water resistant. Due to its nature as a soft container, no backpack will ever be completely waterproof, but military backpacks don’t let water easily.