What defines Class A office space?

According to CoStar, Class A office space is defined as follow:

In general, a class A building is an extremely desirable investment-grade property with the highest quality construction and workmanship, materials and systems, significant architectural features, the highest quality/expensive finish and trim, abundant amenities, first rate maintenance and management; usually occupied by prestigious tenants with above average rental rates and in an excellent location with exceptional accessibility. They are most eagerly sought by international and national investors willing to pay a premium for quality and are often designed by architects whose names are immediately recognizable. A building meeting this criteria is often considered to be a landmark, either historical, architectural or both. It may have been built within the last 5-10 years, but if it is older, it has been renovated to maintain its status and provide it many amenities. Buildings of this stature can be one-of-a-kind with unique shape and floor plans, notable architectural design, excellent and possibly outstanding location and a definite market presence.

Some notable Class A buildings in Boston:
John Hancock Tower: 200 Clarendon Street
Bank of America: 100 Federal Street
Prudential Center: 800 Boylston Street
Federal Reserve Plaza: 600 Atlantic Avenue
Exchange Place: 53 State Street
One Federal Street: 1 Federal Street
One Financial Center: 1 Financial Center
100 Summer Street: 100 Summer Street
International Place: 1 International Place