The central business district (CBD) is considered to offer unique advantages in three functional areas: furnishing specialized and comparison shopping, providing office facilities for the so-called "confrontation" industries, and servicing the small businesses and industries that seek the economies which concentration offers. The chief advantage that the CBD is said to retain in the retail sector is the opportunity for specialized and comparison shopping. The CBD is considered to offer locational advantages, too, for small plants and businesses which produce nonstandard products and are dependent on the use of "external" facilities or services. The remaining large activity for CBDs, Edward Ullman says, is the office function. This is growing in great cities, particularly in New York City which has witnessed a boom in central office construction. Homer Hoyt, a well-known urban planner, reinforces what Ullman says regarding the increasing importance of office buildings in the CBD.