There are two types of outer covers, telescoping top, and migratory top.
Telescoping top is bigger than the Boxes, that way you are able to shift the top frontwards or backwards. The tin on the top of the cover keeps the rain off better than just painted wood does. With a telescoping top you can use the upper entrance on the Inner Covers.
Migratory tops are made for moving hives so unlike the telescoping top this top doesn’t stick out of the sides, this allows you to group hives together closely and ratchet strap them together. This is very helpful when you are moving a lot of hives on an open trailer.
This is a telescoping top. The migratory top doesn’t stick out on the edges like this one does. See Hive Parts for more details on this entire subject or click a link here to find out more about a specific part, Hive topper/Vent box, Inner Cover, Boxes (Brood chamber and honey super together), Bottom Board (specifically a screen bottom board in this picture).