A prescription map is the plan for one application. It stores a target rate for each part of a field, for one product, before the machine moves. People also call it an Rx map. The controller reads it during a map-based pass and tries to hit the number under the machine.
The map is not the field. It is a file that sits on top of the field: a set of areas, and a table of values for those areas. The rate is one column in that table. Other columns may name the product, the zone, or an identifier the display uses to keep the file straight.
What the areas look like
The areas are usually polygons. They may be management zones, irregular shapes that follow soil or yield. They may be a grid of equal squares or hexagons. They may be the dissolved outlines of every place that shares one rate, so a field with five rates becomes five regions, even if those regions are scattered. The controller does not prefer one of these drawings. It needs a boundary for each rate and a number inside it.
A finer grid can hold more detail than a handful of zones. It also makes a larger file, and the machine cannot change rate instantly. Valves and conveyors take a second or two to catch a new target, so a map that flips rate every few feet will be smoothed by the equipment even when the file is sharp.
The file the display reads
On many displays the file is a shapefile. That is a small set of files that travel together: the shapes, an index, a table, and a projection file that says how the coordinates sit on the earth. The rate lives in the table. The column the display expects is not the same on every brand. One looks for a column named one way, another looks for a different name, and the length of the file name can be limited.
ISOBUS equipment can use a different package, an ISOXML task file defined under the ISO 11783 standard. It can carry a rate plan as well. A display that reads a shapefile from a USB drive will not necessarily read an ISOXML file, and the reverse is also true. The prescription is the plan. The file format is the envelope a particular cab expects.
One product is the usual contents of one map. A spreader with two bins can run two products in one pass, and each product still has its own rate layer. Loading the phosphorus map into the nitrogen bin applies the wrong plan, even when both files are drawn on the same field.
Plan, not record
The prescription says what was supposed to happen. The as-applied map says what the controller logged while the machine was moving. The two are easy to confuse because both can be drawn in the same colors, on the same field. One is written in the office, or by the software that built the plan. The other is written in the cab, during the pass. A later comparison is how people see where the machine kept up with the file and where it did not.