A more thorough explanation:

The Squires Doctrine is a rule of the U.S. Patent and Trademark Office that allows a utility-patent claim to incorporate drawings or tables by reference, but only when there is no practical way to express the information in words, and when referring to the artwork is a concise way to communicate the information. This claim is allowed only when necessary, and is not available just for the convenience of an applicant.

An example of a Squires claim is a numerical font designed to be readable in the dim red light inside a submarine. The invention is simple to illustrate with a drawing, and there is no practical way to define the invention in words. Therefore, the claim can incorporate the drawing by reference.