Sashlocks are a combination lock with a latch which is retracted by the motion of a lever and a deadbolt which is retracted by the turn of a key. In general practice, a sashlock is ideal for doors where you have a handle on a lock backplate. They are either low security (2-3 lever) or high security (5 lever).
A 5 Lever Sashlock can be British Standard Insurance Approved, but not always. It is therefore best to check what your Insurance company requires for external doors.
2-3 Lever sashlocks should in principle only be used on internal doors.