A long time ago a controller would watch a screen that was driven by just one radar site, and it had the typical "sweep." With the advent of computer processing of the radar data, engineers figured out a way to combine all that data from several radar sites, so it could be sent to all plan view displays (another name for radar scopes) in a facility, and the controller didn't necessarily know (or care) which radar site he was using. The displays utilized presumably the "best" data from among the radar sites available.
Apparently crunching all that data was way too much work for the computers, so the engineers had to figure out a way to reduce the data to "bite size chunks." Such was the advent of the "selective rejection" method of handling the data. What the computer program essentially does is divide the airspace into "bite size" boxes. These "bite size" boxes are termed "radar sort boxes." Their size is 16 nautical miles by 16 nautical miles, thereby encompassing 256 square miles each. Each box is assigned up to, but no more than, 4 radar sites. The first radar site is called the preferred radar, the second is the supplemental radar, with the 3rd and 4th radar sites being the backup radar if number 1 or 2 are out of service. If you follow the drift here, you'll be catching on to the fact that only one radar site's data is used at any given time and any given location. That's right, one-and-only-one radar's data is used at a time. All the other radar data goes unused, or is dumped into the "bit bucket" as they say in computer programming jargon. That's right...it is thrown out, discarded, abandoned, chucked, unused.
Also, be advised that all of this is taking place inside the bowels of the computer. The air traffic controller has absolutely no control over this internal function. (I know that many people think the air traffic controller can simply "flip a switch" to eliminate VFR radar targets. At the two facilities that I have worked at that is far from the truth...and I never worked beside a colleague who attempted to do such a thing.) By the way, the air traffic controller doesn't see radar sort boxes. These radar sort boxes are strictly an internal thing in the computer program.
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