This one gets settled at the estimate, because it is first in the ground
Almost every other decision on a floor can wait a while. The finish gets called on pour day. Joints get cut after the concrete is down. Sealer, if it happens at all, happens later still. A vapor barrier is the opposite. It goes on the stone before anything else does, so by the time there is a floor to look at, the answer is already permanent.
So it belongs on the written estimate before the base is even finished. When we come out we ask what the building is going to be, not only what it is today, because the bare pole barn that turns into a heated workshop in four years is the case that catches people out.
Which way we would go, building by building
An attached garage gets one. It shares air with the house, it usually gets finished or coated sooner or later, and damp coming up through that slab has somewhere to go and something to ruin.
A heated shop gets one. Warm air sitting over a cool, damp slab is how you end up with water beading on the floor in the spring and rust on everything set down on it.
Any floor you might coat with epoxy gets one, even if the coating is years out. A coating seals the top of a slab. Moisture still working its way up from below then has nowhere left to leave, so it collects under the coating, and when it eventually pushes the coating loose it takes the top skin of the concrete with it.
An open equipment shed with a bare floor, where nothing is getting coated and nothing stored minds a little dampness, is where plenty of owners skip it and we do not argue. That is a real choice rather than a corner cut, and whichever way you go is written on your quote so there is no confusion about what you bought.
Driveways, patios, walks and porch slabs do not get one. They are rained on from above and open to the air on both faces, so there is nothing there to trap.
How the sheeting goes down on pour day
It gets rolled out over the finished stone subbase, not over bare dirt and not partway through the pour. Sheets get overlapped where they meet so nothing is left open between them, and the plastic carries out to the forms on every side. Cover part of the pad and stop, and the uncovered strip is a straight path for ground moisture into the slab.
Reinforcement then goes above it, held up on chairs so the steel sits inside the slab where it does some good. That is worth saying out loud, because the quickest way to spoil a barrier is to drag steel across it and punch it full of holes. Once it is down, it gets treated carefully.
Nothing about the pour changes for you. Concrete goes in, gets screeded, floated and finished the same way it would otherwise. The barrier is invisible from the moment the first load hits it, which is exactly why it has to be right the first time.
If the floor is already poured
This is the call we get most often on the subject. There is an existing slab, there is a plan to coat it, and nobody knows what is under it. A barrier cannot be added beneath a floor that already exists. From there the choices are all on the surface side: a coating system chosen to tolerate moisture coming through, or a penetrating sealer. Both manage the problem instead of stopping it at the source, and it is fair to know that going in.
Before spending money on any of it, find out whether your slab is actually passing moisture. There are simple tests for it that a flooring or coating installer runs as a matter of course, and they cost very little next to a coating that fails in a year. We would rather point you at that test than sell you work you may not need.