A flat rate application puts one target on every acre. The controller is given a single number, in product per acre, and it holds that number for the whole field. If the field is 160 acres and the target is one rate, the pass is planned as that rate times the acres, minus whatever the machine fails to cover.
The phrase is older than GPS. A spinner spreader set to one door opening, or a spray boom set to one pressure and one speed, was a flat rate pass. The operator kept the speed steady because speed was part of the rate. Modern controllers can hold the rate as speed changes, and the pass is still flat if the target never changes with location.
What a flat rate pass can still do
A single rate does not mean a bare machine. Autosteer can drive the lines. A coverage map can show where the boom has been. Section control can shut sections off on headlands, point rows, and waterways. None of that changes the target on the acres that are still receiving product. Those acres all share one number.
The record of the pass can still vary. An as-applied map from a flat rate job often shows small differences from speed, from turning, and from sections switching off. Those differences are what the machine did. They are not a second target that someone wrote for the hill and another for the low ground.
Where the one number comes from
The number is chosen before the pass. Common sources are a field-average soil test, a yield goal for the crop, a figure from an agronomist, or the rate the farm used on that field last year. A composite soil sample, mixed from cores walked across the field, produces one lab result and therefore one rate. The sample is describing the field as a whole.
That is the point of contrast with variable rate application. Variable rate starts from the idea that the field is not one result. A flat rate starts from one result, or from one decision, and applies it everywhere the machine is told to stay on.
The same machine can do either
Most rate controllers that read a prescription map can also hold a constant. The hardware is not what names the pass. The pass is flat when the target is one number, and variable when the target is a surface of numbers tied to location.