If a license code is not defined in a package, but the package contains a license file:

1. Check other versions of this package in the repository
   If the other version contains an identical license file, copy the license code with the description The license file is identical to the file from version {Version}

2. Check repository license files
   If the license is defined with an identical license file, copy the license code with the description The license file is identical to the {Code} license file.

As an example nuget package Microsoft.NET.Test.Sdk

Spec contains only LICENSE_NET.txt:

<package>
  <metadata>
    <id>Microsoft.NET.Test.Sdk</id>
    <version>17.6.3</version>
    <license type="file">LICENSE_NET.txt</license>
  </metadata>
</package>

When the first time the package appears in the repository, a license ms-net-library cannot be resolved automatically. The license code should be updated manually.

index.json

{
  "License": {
    "Code": "ms-net-library",
  },
  "Licenses": [
    {
      "Subject": "package",
      "Code": null,
      "HRef": "LICENSE_NET.txt"
    }
  ]
}

Update Microsoft.NET.Test.Sdk from version 17.6.3 to 17.7.2. Version 17.7.2 as well contains only LICENSE_NET.txt:

<package>
  <metadata>
    <id>Microsoft.NET.Test.Sdk</id>
    <version>17.7.2</version>
    <license type="file">LICENSE_NET.txt</license>
  </metadata>
</package>

The content of LICENSE_NET.txt in 17.7.2 is identical to 17.6.3, so the tool automatically assigns ms-net-library license code for version 17.7.2:

{
  "License": {
    "Code": "ms-net-library",
  },
  "Licenses": [
    {
      "Subject": "package",
      "Code": "ms-net-library",
      "HRef": "LICENSE_NET.txt",
      "Description": "The license file is identical to the file from version 17.6.3"
    }
  ]
}