@mhvk - I guess something like this *could* go in `SkyCoordinate`, too, but that's not what I was saying here: this is for the low-level coordinate/frame objects.  The main reason for this is to make it so there's a standard way for `SkyCoordinate` to know what it needs to copy around as it transforms between various different low-level classes.  This is mainly motivated by the fact that this is *missing* from the current coordinate classes, leading to various ad-hoc techniques that are prone to breakage, like if statements checking if something `isinstance` of some particular coordinate system.  So hopefully adding a standard name that has all the attributes that are needed to specify the frame will get rid of that problem.