The bird grows and the band cannot slip back off. The only way to remove a closed band is to cut it off, which damages the band. Be aware that it is possible for a breeder to make or custom order closed bands with any information they want. So, a closed band only proves that the bird was raised in captivity. However, closed bands are usually reliable sources of information.
Open bands have a seam and can be crimped on the bird at any age.
They can be made of plastic, as well as metals. Imported birds are supposed to have a U.S.D.A. quarantine band, which is a narrow, open band. Sometimes, breeders fail to put a closed band on a baby bird in time, so they must use an open band to identify the bird. If you buy from an honest source, an open band can provide as much information as a closed band. Because they can be removed and possibly transfered to another bird, an open band is never a guarantee.