It is the smallest geographical unit for which the bureau publishes sample data, i.e. data which is only collected from a fraction of all households.
Typically, Block Groups have a population of 600 to 3,000 people.
Every Census Block Group has a unique 12-digit FIPS code.
The Block Group's unique identifier is the 12th digit of the FIPS Code.
For the 2000 US Census, the United States including Puerto Rico had 211,267 block groups, each containing an average of 39 blocks.