Specialist backpacks are used by fire services for wildfire fighting, as well as by rescue services for Search and Rescue. These backpacks are generally very modular, allowing the pack to be reconfigured to the users wishes and are designed to load around the wearers hips. They may include features such as sections for water bladders and specially designed pouches, such as those used to carry personal fire shelters. https://i.pinimg.com/originals/f4/f9/65/f4f965e53d583780a583a38c257fc0bb.jpg
Sports and hydration backpacks are smaller with a profile closer to the body, wider straps and can come with water bladders and hip belts for running, cycling or hiking. Running hydration packs are the smallest and lightest, many under 2 litres and most under six litres. Compression straps across the top of one's body are common as are hip belts. Cycling hydration packs are six to ten litres sitting high on the back. Although daypacks are small averaging ten to thirty litres, all Trekking and Hiking hydration packs are generally the largest and heaviest. Thirty five up to sixty five litres and above are common. https://i.pinimg.com/236x/f3/e9/94/f3e99421e950053c01c28a80d5bccdb7--glitter-lashes.jpg
A daypack is a smaller, frameless backpack that can hold enough contents for a day hike, or a day's worth of other activities. They are not large enough for average wilderness backpacking that use full-sized sleeping bags and backpacking tents, but may be large enough for ultralight backpacking. Padded or unpadded waist straps may be provided to distribute weight across the body. https://i.pinimg.com/originals/71/06/b5/7106b5222162bcb9bc6e8f7bbaf2d0f0.jpg