A premises is defined as any geographically unique location in which agricultural animals are raised, held, or boarded. So if you have hogs in one location, and more hogs in a different location more than 1/4 mile away, you'll need a second number for the second location.
A national animal identification system is necessary to help protect American animal agriculture from disease threats. The ability to find potentially sick or exposed animals early in a disease outbreak is essential to controlling the outbreak quickly. This would allow for rapid traceback of animals in the event of an outbreak, helping to limit the outbreak and minimize the impact on markets.
The identification system may benefit producers in other areas as well, including providing additional marketing opportunities. The NAIS will also help uphold the reputation of the United States as having healthy animals and will promote continued confidence in American agricultural and animal products.
Below is a link to the US Department of Ag and Land Stewardship. It refers to the premise ID program that is currently voluntary in Iowa, but will be required eventually.This link also has great background information about the national ID program and other livestock programs.