Each sheet “placeholder” has a filename, and it refers to an actual schematic. Multiple sheet placeholders can refer to the same schematic file. If you change that file, all references have this new info. Behind the screens KiCad has to work some magic. For example, it results in one schematic file, but with multiple reference designators for each of the instances.
I’m not sure you need to do that. You’re probably contemplating global labels for this.
IMO, it’s much better to use the hierachical labels instead.
What you need to know, is that the hierachical labels need to be defined inside each sheet first. As soon as that’s done, you move up one level and select the tool “Import sheet pin”. That will let you place the labels on the circumference of the sub-sheet. A lot like adding pins to a symbol.
After that, you can route wires between sheets just like between symbols.
A word of advice: Don’t let the appellation sheet make you think a sheet has to contain a lot of circuitry. I have sheets that are no more that a couple of ICs plus their decoupling capacitors.
Oh, and if you find that the contents of a sheet look tiny in the frame, reduce the paper size of the sheet in Page Setup. I use A5 for many of my sheets. The PDF viewer or printer will scale it up on output.
Right now, I have 1 schematic per board, and the four hierarchical sheets I threw together are set up so I can better visualize how that board fits into the whole. Your comment does give me a glimmer of an idea in the back of my head - there are two boards that would probably be better broken up into stages, but I need to get some hands-on time with this feature to better understand it.
The bit about harmonizing the labels turned out to be worthwhile, as I ended up finding and correcting a number of errors along the way.