Impact wrenches have external hex drivers. They are frequently used by by mechanical engineers working with steel, mechanics fitting truck wheels & in the construction of scaffolding & shelving systems. They are sometimes known as nut runners, or occasionally buzz guns.
The key consideration when choosing between impact wrenches is the size of the hex drive (measured A/F or "across flats") to suit increasingly larger sockets for increasingly larger nuts & bolts.
**Battery words** Additionally there is often a choice between single, 2 or 3 speed gears. Most impact wrenches have a variable speed function.