As a mod author, how am I intended to use this? The player could have any number (or none!) of other bz mods installed. Is the idea to list this as the dependency then check for the other mods directly after?

This is purely to enforce mod load order. Basically, if you have a mod that should load after all other mods (eg. DSR/Deadlock Stacked Recipes) then you can add this as an optional dependency, and then if the player uses any BZ mods (even future ones), the mod load order will be enforced, and you won't need to keep updating your dependencies every time i release a new mod.

Thanks!

Hmmm this only works if bz-dsr-bridge is actually loaded though, and if someone is just using a few BZ mods, they probably won't have installed it...

Hmmm this only works if bz-dsr-bridge is actually loaded though, and if someone is just using a few BZ mods, they probably won't have installed it...

no, because bz bridge has all other bz mods as optionals, not as required

Yes, but that doesn't guarantee that the user even has bz-dsr-bridge installed in the first place, unless brevven adds a ~ dependency from each other mod to this one, like Aluminium has https://mods.factorio.com/mod/bzaluminum/dependencies?direction=out&sort=idx&filter=all

This is true. Philosophically, I try to make my mods as modular as possible. For most of them I want them to "just work". As excellent as Factorio's mod support is, it's still not too hard to run into situation where you don't have a dependency installed or enabled, and I wanted to avoid that frustration. Basically solving for the major use case.

So I try to avoid non-optional dependencies in most cases. In 1.1 Aluminum already had a dependency on Foundry, and had some outsized updates in data-final-fixes due to complex interactions with a variety of other mods. It seemed worthwhile to add the ~ dependency in that case.

FWIW, I will eventually update the various modpacks to 2.0, which also add this dependency, to further reduce the number of situations in which someone would need this and not already have it.