Why is there an Output Coordinates Environment setting for Select by Location

[Craig_Eissler_Iceman]

Why is there an Output Coordinates > Environment setting for Select by Location?

I guess I'm not fully understanding because there is NO output dataset.
...only the Input dataset has certain features "selected".

What am I missing?
Thanks,

[DanPatterson]

Select Layer By Location (Data Management)—ArcGIS Pro | Documentation

This tool evaluates a spatial relationship in the coordinate system of the Input Features data source. Set the Output Coordinate System environment to Current Map [Layers] to use the same coordinate system as the current display.

So this would standardize the coordinate systems of the two layers which may differ, but because of projection-on-the-fly, they would be standardized to the coordinate system of the map (to which both layers 'appear' to be in because of p-o-t-f)

[Craig_Eissler_Iceman]

OK thanks, Dan. I think the term "Output" coordinate system was initially confusing me because there is no output layer for this tool. However, what I think they mean, and what you might be saying, is that this could really be considered the "Analysis" coordinate system.

So, is the Default be the Input Layer coordinate system?
Then (and maybe what you're describing below), for display alignment purposes, the data will still project on-the-fly to the Map coordinate system, regardless?

[DanPatterson]

It is the map coordinates system that is used.  For example, if both files are in geographic coordinates and the map is in UTM, then UTM will be used.  If one was already in UTM, then it wouldn't be need to beI projected but the other would so that it matched the map coordinate systems..

I have never liked projection-on-the-fly since it removes a data preparation step that gets masked by "automagic" helper functionality

[Craig_Eissler_Iceman]

hmmm, are you sure? I can't seem to find a Help doc that would substantiate that. But, to be honest, I'm having trouble interpreting the Help docs that appear to address this;
for instance: https://pro.arcgis.com/en/pro-app/latest/tool-reference/appendices/spatial-reference-and-geoprocessi...

[Craig_Eissler_Iceman · Accepted solution]

After further investigation and speaking with Esri Support, they confirmed what we thought; that the term "Output Coordinate System" also applies to outputs that are "dynamic" and not necessarily a new layer; which also means that the "default" output coordinate system will be the coordinate system of the "Input coordinate system" layer; and in cases where there are multiple input layers, it will take the 1st layer's coordinate system.

Of course, you can override the default with something else. That said, for best results -- as we all know -- the coordinate system of all tool layers, as well as the Map itself, should be in the same projected coordinate system.