A trademark application filed based on actual use must include a specimen showing use of a trademark in commerce that can be regulated by US Congress. There must be a specimen for each trademark class that an application covers.
For applications filed based on an “intent to use” a trademark for certain goods/services, the filing of the specimen is not done at the time the application is filed, but is instead filed later via either an Amendment to Allege Use or a Statement of Use.
Specimens should show how the mark is actually use in commerce and should not be digital mockups or renderings.
For goods, some examples of specimens could be a photograph of the mark appears on a tag, label, packaging or at the point of sale (e.g., on a website product page that has an “add to cart” feature). Advertising materials are not proper specimens to identify the sale of your own brand of goods.
For services, specimens can include advertising materials, such as websites, pamphlets, tv ads, etc.