cosmos-sdk/docs/core/handlers.md

Handlers

A handler takes a context and a message and returns a result. All information necessary for processing a message should be available in the context.

While the context holds the entire application state (ie. the MultiStore), handlers are restricted in what they can do based on the capabilities they were given when the application was set up.

For instance, suppose we have a newFooHandler:

func newFooHandler(key sdk.StoreKey) sdk.Handler {
    return func(ctx sdk.Context, msg sdk.Msg) sdk.Result {
        store := ctx.KVStore(key)
        // ...
    }
}

This handler can only access one store based on whichever key its given. So when we register the handler for the foo message type, we make sure to give it the fooKey:

app.Router().AddRoute("foo", newFooHandler(fooKey))

Now it can only access the foo store, but not the bar or cat stores!