"Fiber diapers" refers to a type of absorbent underwear made from natural fibers such as cotton or bamboo. These diapers are used for babies or individuals who wear them for bladder or bowel control. Full definition
Perhaps it's because cloth and natural fiber diapers provide better airflow for a baby's bottom. (easybabytote.com)
Tip: You may wash your natural fiber diapering products with your other white laundry for the prep cycles, as long as you use a cloth diaper safe detergent. (thirstiesbaby.com)