Cmd creates a service worker at build time using sw-precache that automatically caches the app shell.

https://docs.sencha.com/cmd/7.1.0/gu..._web_apps.html

sw-precachelink goes to
https://github.com/GoogleChromeLabs/sw-precache

which states:
sw-toolbox and sw-precache are deprecated in favor of Workbox. Please read this migration guide for information on upgrading.