The startup lifecycle

Foundry starts a world in four stages. registerSystem and registerModule take a callback for each.

registerSystem({
  id: 'my-system',
  onBeforeInit: () => {},   // before any CONFIG mutation
  onAfterInit: () => {},    // init, after the mutations
  onI18nInit: () => {},     // languages loaded
  onSetup: () => {},        // every package loaded
  onReady: async () => {},  // the world is open
});

Register only the ones you use. A callback you leave out registers no hook at all.

What each stage is for

init is where you write CONFIG: data models, document classes, sheets, the initiative formula, settings. registerSystem does most of it for you from the options you pass. onBeforeInit runs first, before anything changes, and is the usual home for globalThis.<systemId>. onAfterInit runs last, and is where you register settings.

There is no game.user yet, no game.actors, no canvas.

i18nInit is the first moment game.i18n works. A label you localize during init comes back as the key you passed in, because the language files have not loaded, and players read that raw key on screen. Translate CONFIG labels here, once:

onI18nInit: () => {
  for (const ability of Object.values(CONFIG.MY_SYSTEM.abilities)) {
    ability.label = game.i18n.localize(ability.label);
  }
},

Doing it once here beats calling localize on every render.

setup runs after every package has finished its own init. A setting you registered during init can now be read, and a module can see what the system around it registered, so a module that extends a system does that work here rather than racing it in init.

Compendium packs are available. World documents are not.

ready runs with the world open, so game.actors, game.scenes, game.user and the canvas all exist. Migrations go here, guarded, because they write to the world:

onReady: async () => {
  if (!game.user.isGM) return;
  await migrations.run();
},

None of the callbacks are gated for you: onReady fires on every client, so the GM check is yours to write.

The cost of doing everything in ready

Everything exists by then, but the user watches the work happen. CONFIG written during ready lands after the sidebar has rendered against the old values, and a sheet that opened first opened wrong. Each stage is the earliest point its work can succeed.