When asked to provide a specimen, you will have to prove that the trademark is actively used in the US market. This proof depends on the type of your trademark.
In the case of evidence for physical goods, the connection between the product and the trademark must be visible. That means you must submit a photo, screenshot, or other possible representation of the trademark displayed on goods, labels, tags, packaging, sales displays or web pages where consumers can purchase the goods.
With services, the trademark must be directly associated with the service offered. The trademark can be used in advertising materials, on a website, or other business documents, including signage on invoices showing a direct association between the trademark and your services. For the taxi service, a visible sticker on the car would also be acceptable.
If the USPTO rules the specimens as insufficient, it may cause complications in the trademark registration process.