The conversation happens before the concrete is booked, not after
The sequence matters more here than any single ingredient does. Somebody calls the plant, gives them the strength for what is actually being poured, and tells them the load is going through a line pump. Everything else follows from that one sentence. Leave it out and the plant builds for a chute, because that is the default and they had no reason to think otherwise.
A mix meant to slide down a steel trough into an open form has no friction to speak of working against it. That same strength, batched the same morning, turns into something else entirely the moment it has to be pushed a hundred feet along a rubber wall. Nothing about a pump mix is exotic. It simply has to be designed for the trip it is really making.
On our own pours we make that call ourselves, well ahead of the first truck being booked. In this climate most of what we pour gets specced as an air-entrained mix at 4,000 PSI, built to take Virginia freeze-thaw, and the same ticket says the load is going through line. Those are two separate items and neither one stands in for the other. Both belong on the order.
What the plant does once it knows
Their mix design people take it from there, and it is genuinely their trade rather than ours. Sand, stone and paste get proportioned against each other differently. The top aggregate size gets chosen against the hose it is going into rather than against a chute, because everything suspended in that load has to make the trip with room around it. Admixtures do whatever the design needs them to do.
The one thing a plant cannot do is design around a fact nobody handed them. That is the whole of the answer. Same materials, the same strength written on the ticket, a different set of proportions arrived at on purpose.
You will notice there is no slump figure on this page, and that is deliberate. The right number falls out of whatever design the plant built against the strength and the conditions you ordered for. The shape of it is all you need from us: concrete too stiff has to be forced and packs against the inside of the line, concrete too loose comes apart on the move, and the middle gets hit at the batch plant with proportions rather than at your site with a garden hose. Ask your supplier for the number, and tell them it is being pumped.
Pour morning, when the first truck backs in
The ticket comes off the truck and gets read before a shovelful goes into the hopper. A load that does not match what was ordered gets sorted out right there, between the driver, the plant and whoever is running the machine, while the concrete is still in the drum. Ten minutes spent at the hopper is cheap. The same problem discovered forty feet down a hose is the rest of the morning.
The other thing that happens ahead of concrete is priming, and it is worth separating from the mix question because it gets blamed for the same failures. A wet slurry runs the hose first to coat the inside wall so what follows slides instead of grabbing. The first thing out the far end is that slurry, and it gets caught somewhere rather than placed.
After that the load only has to keep moving. Concrete standing still in a hose stiffens the same way concrete standing still anywhere else does, which is why truck spacing gets talked about at the same time as total yardage rather than left to sort itself out.
The moment somebody reaches for the water valve
Here is where the expensive mistake lives, and it is almost never made by anybody careless. One load shows up tighter than the last, three people agree it will never travel, and the valve is right there at eye level on the back of the drum.
Loosening a load with site water is a trade, and it is a bad one. You get workability now and you pay for it out of the strength you already ordered, permanently, with no way to put it back. That is the usual reason a finished slab comes in under what the ticket said, and nobody finds out for years. What eventually shows is a top that powders under a broom, or scaling that lifts the surface across a few winters. A pump does not change that arithmetic. If a load truly will not travel, the answer is the plant's phone number, because a supplier has ways to fix a stiff truck that do not come out of the concrete.
That is also the honest answer to the worry sitting underneath this question. Pumping does not weaken concrete. Watering a load down so it will pump does, and so does watering one down so it will barrow.