The only meaning of backlog, known to our dictionaries is a large log of wood forming the back of a fire on the hearth. But that is certainly not the meaning it is intended to convey in:

The most important step is to eliminate a very heavy backlog of orders on the manufacturers' books.

" Backlog " is perhaps more picturesque than " arrears " but it is not so intelligible, at any rate in this country. To us the G.P.O.'s " Waiting list of applicants " is clearer than the American Telephone Company's way of putting the same thing, " Backlog of held orders ".