The "Day Pack" is a lightweight backpack worn when a soldier is expecting to operate independently an entire day, providing water, medical supplies, food, ammo, and equipment for 24 hours without being too large. They tend to have a lot of "Ports;" little pieces you can clip extra bags onto without them falling off, like something for a canteen or a cellphone pouch.
Or they can go with a "Cross-strap," which is lower profile but less efficient. It's commonly used with "Freerunners" because the one strap can be pulled tight, as not to swing and throw them off a building.
