Nodejs require module starts with '#'

From Nodejs docs https://nodejs.org/api/modules.html#all-together

require(X) from module at path Y
1. If X is a core module,
   a. return the core module
   b. STOP
2. If X begins with '/'
   a. set Y to be the filesystem root
3. If X begins with './' or '/' or '../'
   a. LOAD_AS_FILE(Y + X)
   b. LOAD_AS_DIRECTORY(Y + X)
   c. THROW "not found"
4. If X begins with '#'
   a. LOAD_PACKAGE_IMPORTS(X, dirname(Y))
5. LOAD_PACKAGE_SELF(X, dirname(Y))
6. LOAD_NODE_MODULES(X, dirname(Y))
7. THROW "not found"

What does no.4 refer to?
What does it mean for a module to start with #?

LOAD_PACKAGE_IMPORTS(X, DIR)
1. Find the closest package scope SCOPE to DIR.
2. If no scope was found, return.
3. If the SCOPE/package.json "imports" is null or undefined, return.
4. let MATCH = PACKAGE_IMPORTS_RESOLVE(X, pathToFileURL(SCOPE),
  ["node", "require"]) defined in the ESM resolver.
5. RESOLVE_ESM_MATCH(MATCH).

This is the LOAD_PACKAGE_IMPORTS but it looks enigma to me.

Solution

The character "#" at the beginning of an import specifier refers to an imports field defined in package.json.

Imports fields are useful to alias file names or other dependencies that are only accessed inside a package.

Considering the example in the documentation:

  "imports": {
    "#dep": {
      "node": "dep-node-native",
      "default": "./dep-polyfill.js"
    }
  }

When a module in this package contains an import statement like

import dep from "#def";

This tells Node.js to import the package "dep-node-native"; and it instructs other build tools (for the browser) to use the local file "./dep-polyfill.js".