The exact reference designator U, R, L, D, etc. is not relevant to KiCAD.. they are for you to be able to discern by one look to what group a particular device belongs to.
'U' usually stands for Integrated Circuit and depicts a complex device which contains electronic devices in a monolithic assembly that you can place on your board - a battery holder is something else.
Please see this list to get you started (and yes, you can make up your very own designators as you go along, no one is stopping you if it helps you, just stick with the more common ones if you ever want to share your work with other people):
Also, as @Andy_P is telling you, KiCAD has a certain way to define power related nets/symbols for your schematic drawing phase that should not be turning up in the layout as a real populated device, but are there only for the ERC/DRC/easier schematic reading - and as far as PCBnew should be concerned - do not exist really.
You must not use these options for real devices that are in your circuit for delivering power ever.