When do I Change/Assign, OR Define the CAD data spatial reference/coordinate system?

Question

See two screenshots below; when referring to whether I need Change/Assign or Define the CAD data spatial reference/coordinate system?

When I Validate (see below-left), I click on "Change", then it appears to want to "Assign"; so obviously, these two terms are interchangeable in this Context ...but the terms seem possibly conflicting, unless what I'm really doing is "Changing the coordinate system by Re-assigning it"..?

And, yet, if I'm in the Georeferencing ribbon (below-right), and perform it here, it's referred to as "Defining" and utilizes the Define Projection tool which I am familiar with, of course.

...so, are these two approaches doing the same thing or not?
Perhaps, yes, but under different scenarios? If so, when would I choose one over the other?

Thanks!

Craig_Eissler_Iceman · Answer

Thanks for your thoughts on the terminology.

However, when a CAD file WITHOUT a Coordinate System defined is added to the Map and I attempt to assign one via Catalog > Properties, the option to do that is called, "Change Coordinate System". ...But, what am I changing it from if there was nothing?

...Or, is there actually something; in other words, does ArcGIS automatically, initially, assign a default coordinate system to match the Map coordinate system or something? For example, if it was a new blank Map, then it would be WG84 Web Mercator (auxiliary sphere), so I would be "changing" it from that..?

As for the elusive PRJ file, I do not see one associated with the CAD data I have WITH geospatial coordinates. However, if I convert that to a GDB, then to a SHP -- and delete the PRJ -- then convert back to a GDB, and back to CAD -- it DOES exist. ...Any explanation for this?

RTPL_AU · Answer

@Craig_Eissler_Iceman  
Fundamentally nothing - just different teams working on different parts calling it different things I think 🙂

If the CAD data has embedded coordsys AND it is correct then you can use it as is. I prefer to double check and make sure.

If the geometry is correct you can add CAD data to a feature class or feature dataset without giving it a coordsys first. Just because you can (not do it) doesn't mean you should (not) do this....

There should be a .prj & .xml for the .dwg/.dxf once you've given it a coordsys. FGDB doesn't have a standalone .prj but it is embedded in the data. You can set a coordsys folder-wide using an ESRI_CAD file.

https://pro.arcgis.com/en/pro-app/latest/help/data/cad/about-cad-coordinate-systems.htm

Craig_Eissler_Iceman · Answer

Thanks, but the question was really "what is the difference between assigning a coordinate system in Catalog/Properties AND defining a coordinate system with the Define Projection geoprocessing tool?

But, in response to your reply: IF the CAD data has a geospatial coordinate system, why would I have to do anything?

And, I'll also add:
if I'm reading the Help Docs correctly, they all indicate that a PRJ (coordinate system) file MUST be assigned before you can do anything. ...I know what a PRJ file is because I see them associated with Shapefiles, but, after assigning/defining a coordinate system (see original question), I do NOT see a PRJ file anywhere associated with the CAD Data or with a converted GDB Feature Class. ...What is the explanation for this?