UMD: UMD builds can be used directly in the browser via a <script> tag. The default file from Unpkg CDN at https://unpkg.com/vue-i18n is the UMD build (vue-i18n.js).

CommonJS: CommonJS builds are intended for use with older bundlers like browserify or webpack 1. The default file for these bundlers (pkg.main) is the Runtime only CommonJS build (vue-i18n.common.js).

ES Module: starting in 8.11 VueI18n provides two ES Modules (ESM) builds:

• ESM for bundlers: intended for use with modern bundlers like webpack 2 or Rollup. ESM format is designed to be statically analyzable so the bundlers can take advantage of that to perform "tree-shaking" and eliminate unused code from your final bundle. The default file for these bundlers (pkg.module) is the Runtime only ES Module build (vue-i18n.esm.js).
• ESM for browsers (8.11+ only, vue-i18n.esm.browser.js): intended for direct imports in modern browsers via <script type="module">.