infocom-systems-design/node_modules/@zenuml/core/docs/module-vs-main-in-package-json.md

The module field is not officially defined by Node.js and support is not planned. Instead, the Node.js community settled on package exports which they believe is more versatile.

For practical reasons JavaScript bundlers will continue support the module field. The esbuild docs explain when to use module as well as related fields like main and browser.

• main is the standard field for all packages that are mean to be used with node. It is reasonable to expect that the file path in this field is a valid CommonJS module.
• module is a non-standard field that is used to integrate ECMAScript modules into node. It is reasonable to expect that the file path in this field is an ECMAScript module (esm).

For @zenuml/core, the main field is set to dist/zenuml.js and the module field is set to dist/zenuml.esm.js.