What is this upside-down silver key icon

I inherited a SQL Server 2005 3rd party server and database to look into to be able to generate reports and dashboards on.

On one table, I found a new icon: a blue-silver upside-down key, alongside my other primary and foreign keys.

When I right-click it, the context menu gives me the option to "Script Constraint as", but when I try to "Modify" it, it opens the column design form (like primary keys), and yet, it appears in the "Indexes" section, not constraints.

Someone knows what it is called and what it is used for?

Thanks

-Philippe