A more thorough explanation:

The functionality doctrine is a rule in trademark law that says a functional product feature cannot be used as a trademark. A product feature is considered functional if it is necessary for the use or purpose of the product or if it affects the cost or quality of the product. The goal of the functionality doctrine is to prevent companies from controlling a useful product feature without going through the patent process, which would inhibit legitimate competition.

One example of the functionality doctrine in action is the case of Valu Engineering, Inc. v. Rexnord Corp. In this case, the court decided that certain conveyor belt shapes were functional because they increased the performance of the conveyor belt over alternative designs. Therefore, these shapes could not be considered a trademark.

Another example is the case of Qualitex Co. v. Jacobson Products Co. In this case, Qualitex had trademarked the color of their dry-cleaning press pads. However, the court decided that the color was functional because it helped customers identify the pads and ensure they were using the correct ones. Therefore, the color could not be considered a trademark.

These examples illustrate how the functionality doctrine prevents companies from monopolizing useful product features and allows for fair competition in the marketplace.