Thanks for the explanation John.
The point I was trying to correct in what another poster wrote was, that just because a plane does not have OUT does not prevent that aircraft from being seen on IN. Several months ago I spent several hours circling around above my airport doing engine break in. It gave me a good bit of time staring at the traffic when not keeping the top side pointed up and looking out the window for traffic. There were many aircraft that I knew and was surprised that they had OUT. I figured out that they did not have OUT, indicated by the fact that no tail number was displayed. I thought that this was R. From your explanation I now know what R is. That said, my correction to the other poster saying that you do see traffic that has no OUT is correct.
Without ADS-B Out, what you see is other aircraft that are equipped and their traffic. If you are also their traffic, you will see a ghost target at your own location. Many systems will eventually figure that out and suppress the ghost, but on initial detection, you will often get a target alert (Traffic less than a mile, same altitude!) which is for your own aircraft. What you don't see is
YOUR traffic, that is traffic near you, but not simultaneously the traffic of a client. Some of the other aircraft traffic may also be your traffic, but if your altitude is more than 3500 feet above or below the client aircraft's traffic, the ground station will not generate a Mode C based TISB for them. Furthermore, before two days ago, the FAA considered aircraft that are equipped with ADS-B Out but an earlier version as clients, but these clients do not have a means of indicating whether or not they have ADS-B In, so as a courtesy, before the rule kicked in. ground stations would generate a TISB on the same frequency as the ADS-B Out for these earlier installations. Most of the early airline aircraft had these ADS-B Out systems if one was installed. This made for more TISB on 1090 MHz than was actually needed because these aircraft did not have any sort of ADS-B In. Now that the airlines have complied, most of these aircraft are set to indicate they do not have ADS-B In of any sort, so ground stations no longer generate a TISB for them. So my point is that what used to generate TISB, no longer does at the major airports or airports with significant air carrier service.
So if you are orbiting above an airport while you do a break in of your new engine at 3500 feet above the airport, you will be a target of most equipped aircraft flying into or out of the airport and have a descent indication of traffic below you. You will likely see their traffic up to 500 feet above you when they are departing or arriving. But if you are orbiting at 4000 feet above the airport, most of their traffic will not be your traffic anytime they are within 500 feet of the surface. So any of your traffic descending into the area are not going to show up as their traffic. This is just intended as an example that dependence on another aircraft to "in effect" light up your traffic is very problematic. The misconception is that just because you see tons of traffic, that the traffic is in fact also your traffic. It is not.