What feature distinguishes a vertical layout on California driver licenses?

The feature that distinguishes a vertical layout on California driver licenses is that it is specifically for persons under 21. This design choice helps to easily identify individuals who are not of legal drinking age, making it simpler for establishments to enforce alcohol-related laws. The vertical layout serves a clear purpose in promoting compliance with age restrictions, ensuring that employees and law enforcement can quickly assess a person's eligibility to purchase alcohol or enter age-restricted venues.

This differentiation by layout is an important aspect of the driver license design in California, as it reinforces public safety and helps to prevent underage drinking. The other options do not accurately reflect the intended purpose of the vertical layout, which specifically targets identification of those under 21 years of age.