According to the official documentation, the related parameter is:
A comma-separated list of accounts related to the content of the shared URI.
But what exactly does it do? When the tweet is sent, the added twitter accounts are never included within the tweet message. And they do not receive a notification either about the tweet.
You will only see the related accounts after you've interacted with the tweet button.
Suggest additional Twitter usernames related to the Tweet as comma-separated values. Twitter may suggest these accounts to follow after the user posts their Tweet. You may provide a brief description of how the account relates to the Tweet with a URL-encoded comma and text after the username.
User hits the button, user posts tweet, user sees the accounts you added in the related field.