@NormanDunbar
Things have probably changed since the 90s and phones etc possibly run off a globalised co-ordinate system???
But from memory co-ordinate systems involved a geoid which defined the locally idealised model of the shape of the earth and then the map systems under that which defined the conversion from 3d globe to 2d map surface. But that could be wrong.
Because the earth isn't a perfect sphere you chose the system that is most accurate locally.
I think we worked with the Australian Map Grid 1984 based on a particular geoid. I believe there was a transition going on to a new geoid/map grid at the time hence govts using different systems.