Specimen

Not a mockup or a logo file. For goods it must show the mark on the product, packaging or point-of-sale display. For services it must show the mark in advertising for those services. Bad specimens are a leading cause of Office Actions.

A specimen is evidence, not artwork. The USPTO is asking you to prove the mark is doing a job in the real world, and the most common reason applications stall is an applicant sending a logo file or a mockup instead of proof of use.

1. For goods, photograph the mark on the product, its packaging or its point-of-sale display. A shipping label or a product listing page usually works.
2. For services, show the mark in advertising for those services - a website page that describes and offers them is the standard example.
3. Check the mark in the specimen matches the mark you applied for. Drift between the two is a refusal waiting to happen.
4. Make sure the specimen shows the goods actually being offered for sale, not merely displayed.

A founder submits a clean logo on a white background. It is refused, because it shows the mark existing rather than the mark being used. A screenshot of the product listing with an add-to-cart button would have passed.