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

[Craig_Eissler_Iceman]

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!

[RTPL_AU]

My read of it is:
1: If DWG is geolocated correctly and matches a known coordinate system - Assign a coordsys in Catalog view before adding the data to a project.

2: DWG is non-geolocated / in weird units / local grid / etc then do not assign a coordsys in Catalog before adding it to a project / map as it will fly off to the never-never and/or Pro will have a fit.  Bring it in clean and then follow the georeferencing process and assign the correct coordsys in that process. That way you will not have to try to override any assumptions Pro has tried to make. 

Of course - it is always best to have a good CAD team on the other side that follow decent CADD standards and data operability processes (use layers, XREFs, etc properly and so forth). This doesn't happen often... 
If the accuracy & repeatability of the CAD data location is critical then you'll have to work with the project owner to come up with a project-wide transformation or georeferencing process to make sure locations are accurately transported through the various data consumers. 
This can be as simple as adding preferred georeferencing marks in a CADD Layer to be used by everyone in the GIS team to bring the data in.

[Craig_Eissler_Iceman]

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?

[RTPL_AU]

@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]

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?