"Z" is a designation which means a steel shield covering the side of the bearing (for most brands). "ZZ" simply means that the bearing has a shield on both sides. Normally, the shield itself will be branded as "608 Z" even though there is a shield on both sides of the bearing. This is for practicality in manufacturing. It is much more economical to produce all of the shields stamped as "608 Z" than it would be to manufacture the exact same component with two different stampings on them. Some manufacturers supply bearings with removable shields, whilst others have shields that are crimped in place. Neither is usually better than the other unless you intend replacing the lubricant or cleaning the bearing from time to time.