What is the meaning of groupName within mailbox?

[Jimmy89]

Example input: "Composers:\"Bach, Sebastian\" <[email protected]>, [email protected] (Mozzie);"

This returns the trimmed output:

[
  {
    type: 'group',
    name: 'Composers',
    addresses: [
      {
        type: 'mailbox',
        name: 'Bach, Sebastian',
        address: '[email protected]',
        groupName: null
      },
      {
        type: 'mailbox',
        name: null,
        address: '[email protected]',
        groupName: null
      }
    ]
  }
]

The variable groupName is not documented within the readme and it also seems to stay null. Looking at the source code I would expect it to repeat the Composers group there, but otherwise I have no clue. Could anyone inform me what this variable implies?