Suppose I decided my house was the centre of everything and I wanted to define a coordinate system based on my house. I'd call this the Baz2017 Geodetic CRS. Now I need to define the Prime Meridian for Baz2017, and that will be the line going through the exact centre of my front door. That is now the Baz2017 Prime Meridian. Next I'll measure the shape of the earth... okay, its 1276 furlongs in radius (I may be way off here, but the Baz2017 Geodetic Consortium is not known for its accuracy) and the earth is clearly banana-shaped with various parameters, so that defines the Baz2017 Ellipsoid (which is really a Bananaoid, but no matter).
The Baz2017 CRS is valid for the whole world, and there's an EPSG code for that, Area of use EPSG 1262, so we'll use that. If Baz2017 coordinates were only valid for a range of 200m from my house, I'd have to define a new Area-of-use, the Baz2017 Area-of-use, and try and get an EPSG code for that.
So here you can see I've used "Baz2017" several times to refer to different components of the coordinate system, in the same way that WGS84 is used formally in your diagram. Informally, I can say "My address is (0,0) in Baz2017 coordinates" and I'm referring to the CRS as a whole. If I want to refer to the ellipsoid or other components, I'd probably be more explicit.
Do not use Baz2017 coordinates for navigation.