Class A buildings are the most prestigious of buildings–competing for premier office users with rents above average for the area. Class A buildings have high quality standard finishes, state of the art systems, exceptional accessibility and a definite market presence.
Class B buildings compete for a wide range of users with rents in the average range for the area. Finishes are fair to good for the area and systems are adequate, but the building does not compete with Class A at the same price.
Class C Buildings competing for tenants requiring functional space at rents below the average for the area.
I can add from my own experience that Class A buildings many times will have a restaurant, bank, 24-hour manned security, covered parking, etc. Class B will have some of the amenities of Class A, but not all of them. Class C buildings (greatest value, fewest amenities) may possibly have a deli, video surveillance, fewer floors, etc. As a business owner, I would not take the classifications too seriously as they are somewhat subjective, but good general guides to for tenants to know what to expect before they walk into a building.
What is more important than the Classification of a building is how well the amenities, location, curb appeal match what you want for your business. A building that has all you want at a competitive market rate is always a Class A deal!