Custom data can be added to the clipboard using the Clipboard API during a copy event, allowing developers to modify or augment the copied content programmatically. This feature can be used to hide additional data within the copied content that isn't visibly selected by the user. For more details, see Mozilla's Clipboard API documentation: Clipboard API - Web APIs | MDN.
Clipboard Hacker is a tool for dealing with this hidden data.
You can use the tool for free at: https://www.clipboardhacker.com/