An as-applied map is the log of a pass. While the machine moves, the controller records where it was, what rate it was commanding, and often the speed, the swath width, and which sections were on. Drawn back on the field, that log is the as-applied map. It is the record. The prescription map is the plan that may have been loaded beforehand.
The two files can look alike. Both are shapes on a field, colored by rate. They are written at different times, by different things. The prescription exists before the engine starts. The as-applied file grows row by row during the pass, and it is finished when the operator ends the job.
What gets logged
A typical log has a position, a time, and a rate. Many controllers also store the target they were chasing, so the file can show both the rate that was asked for and the rate the system reported. Speed matters because application rate is tied to how fast product leaves the machine and how fast the machine covers ground. A section that is off records a zero, or a gap, which is how section control shows up in the picture.
The path is rarely a set of clean polygons. It is a ribbon following the swath, with overlaps where two ribbons cross and with holes on the headland where the boom shut off. Turns are messy. The rate can lag the target when the controller has just been handed a new number and the conveyor or the valve has not caught up.
Why it is not a copy of the plan
Even a careful pass departs from the prescription. The machine cannot change rate in the same instant the polygon changes. GPS can sit a few feet off the line the map assumed. A bridge in the bin, an empty bin, or a stopped belt show up as a stretch below the target. Overlap on a curve puts extra product on ground the prescription counted once. A flat rate job has an as-applied map too. It will not be a perfect sheet of one color.
Sensor-based passes often have no prescription at all. The as-applied map is then the only picture of the rates, because the numbers were chosen on the move. Map-based passes have both files, which is what makes a later comparison possible.
What people use the record for
The log is how an operator, a co-op, or an agronomist sees coverage: skipped strips, overlaps, and the rates that were actually commanded. Totals from the as-applied file are the amounts the controller believes went out, which can be checked against what left the tender. The file is also the start of a record for that field in that season. It does not, by itself, say whether the plan was a good one. It says what the machine did with the plan it was given.