It’s just the scale of the object available in the material. If you apply the material to an object whose scale is changing, the material will now react to the object’s scale like so:
In this example, the R (X) stays at 1, but GB (YZ) goes down to 0, so the object turns red.
If you plugged the object scale into Emissive Color and cranked up the scale above 1, the object will glow. If the object is scaled up in Z, it will glow blue:
Essentially, object’s scale is a material vector param.