What is the difference between no referrer and other referrers (in the tracking point violations table)?

If an impression has a hidden referrer we categorize it as having no referrer.

In the tracking point violations table, we show the top 10 worst performing referrers.

Additional referrers violating the quality metrics in question will be categorized as other referrers.

How did we do?

What is a hidden referrer or a generic referrer?

When is a tracking point considered to be violating Geo Compliance/Transparency/Viewability?