This explains visa number usage, spillover of visa numbers into other preference categories, and the visa number backlog.
Preference Categories
The primary reasons for entering the immigration process in the U.S. are family and employment-based. To manage this process, family and employment-based immigration is broken into preference categories. Congress sets limits on how many individuals may immigrate to the U.S. each year (i.e. visa numbers) and also sets limits on how many individuals may immigrate within each preference category.
This visa category applies to people who wish to perform services in a specialty occupation, services of exceptional merit and ability relating to a Department of Defense (DOD) cooperative research and development project, or services as a fashion model of distinguished merit or ability.