The Postal Service has two basic subclasses for letters and  First Class (originally rapid, non-bulk, and sealed against inspection) and Standard A (originally deferrable, bulk, and not sealed against inspection).
The postal service has two subclasses of letters.
The postal service has seven subclasses of letters.
A crowd is walking to the club
There are three people.
A man is shopping.
A young girl is on the moon.
A man flying his jet.
A woman is looking at the camera.