When is a session or contact bucketed under Other campaigns in HubSpot?

When a visitor or contact comes to your site from a paid social post that’s associated with one of your HubSpot campaigns
When a visitor or contact comes to your site from any asset that’s associated with one of your HubSpot campaigns
When a contact is imported to HubSpot
When a visitor or contact comes in from a tracking URL you’ve created in HubSpot

The correct answer is:

When a visitor or contact comes in from a tracking URL you’ve created in HubSpot.