Where you actually see one working
The word turns up on plan sets, in a general contractor's schedule, and on any job where the concrete has to end up above the ground it was delivered on. An upper deck on a building already framed. The top of a tall wall. A large commercial pour where the placing point keeps moving and nobody wants to be relaying hose all day. In each of those the arm is doing work nothing on the ground can do, because the obstacle is not sitting beside the pour. It is under it, or standing in front of it at height.
Mechanically it is a pump like any other. What is different is the delivery. Instead of hose walked out by hand, the concrete travels through fixed pipe running along the folding sections of the arm. That pipe is the slick line, with a length of flexible hose at the tip so the placing end can still be steered. The operator works the arm rather than the route.
What the arm asks of the ground and the sky
An arm loaded with concrete has to be braced, so the truck puts outriggers down and spreads its weight onto a few pads. Every one of those pads wants firm, reasonably level ground under it. On a wide open commercial site that is easy. On a residential street with cars along both sides it often means the setup ends up in a travel lane, and taking a lane means arranging traffic control with the locality ahead of the date.
The sky matters just as much. The arm has to clear whatever sits above the pour and above the path it swings through, so service drops, phone and cable lines, and limbs grown out over the street all get looked at first. None of that is a criticism of the machine. It is the price of going over the top of something instead of around it.
We do not own one, and we will say so on the phone
Worth stating plainly, because it is the sort of thing outfits fudge. We run a trailer-mounted line pump on 2.5 inch and 3 inch line, with our own operator and our own hose guy on it. We do not own a boom pump, and we will not dress ours up as one.
So if the concrete has to land on an upper deck, on top of a tall wall, or out past a building that is already up, the honest advice is to book somebody who owns that machine. You will get that advice from us on the first call, not after ready-mix is on the way. When it has to travel around the side of a house instead, in through a gate, or down into a hole, that is squarely our work. Which direction the concrete has to travel is the whole question, and the full comparison between the two machines has its own page on this site.