The Ref ID is a special type of project and user property you can use to map users and projects with a different database.
Projects can have a Name they're known for by everyone in your team and a different Ref ID that matches the same entry on a different system, such as your billing software, or another document, such as an SOW or a PO.
Users' names are loaded from Slack or Microsoft Teams, and sometimes users use names that will make it hard to understand those monthly reports. Use the Ref ID to match each user with the official documents and payroll software.
When exporting your data, you can include this data as custom fields.
See how to export your data for more details.