What happens at the plant
The mix design gets decided here, not on the road and not at the pour. A batch plant weighs each ingredient against a written recipe for the strength, the exposure and the slump the order calls for, then combines them in a set sequence. That recipe is what turns a pile of cement, sand and stone into a specified 4,000 PSI mix instead of a guess.
This is also where anything unusual about the order gets built in from the start. Air entrainment for exterior work, fiber, an accelerator or a retarder, a different top stone size for a tighter line. All of it goes into the batch, not added later at the job.
The plant is not the truck
People say the plant when they mean the truck, and the two do different jobs. The plant proportions and mixes from raw materials, once, against a written spec. The mixer truck's drum keeps turning after that so the load stays workable on the ride over, but it is not mixing anything new into the concrete. What left the plant is what shows up at the site.
That distinction matters when something about a load looks off. A stiff or loose truck is usually a batching question, decided back at the plant before the truck ever pulled out, not something that happened on the road.
Ordering from the plant
We do not run a batch plant ourselves. We order ready mix from a supplier's plant against a written spec, the same way any contractor does, and we bring our own trailer mounted line pump and crew to place it.
On our own work, telling the plant the strength, the exposure and any admixtures is our conversation to have before anything is ordered. When we sub the pumping for a job we are not pouring, that conversation belongs to the contractor and the supplier who booked the concrete.
Loads also get batched to fit a full truck. Ready mix is ordered in ten cubic yard loads rather than by the odd cubic foot, and a plant will round a job's yardage up to fill that load rather than send a half empty drum. That is a batching reality worth knowing before you plan a small pour around an exact number.