In Atlanta, you'll often hear people refer to a neighborhood or event as either "ITP" or "OTP." This terminology is short for "inside the perimeter" or "outside the perimeter." The perimeter of Atlanta is defined by I-285, which forms a large ring around the city.
Several Atlanta suburbs are still inside of I-285, even though they are not part of the City of Atlanta.
Often times the terms are used as playful banter among friends, with many city-dwellers claiming they'd never venture "OTP."