What is Borough, Block And Lot?

Legal Definition
Borough, Block, and Lot (also called Borough/Block/Lot or BBL) is the parcel number system used to identify each unit of real estate in New York City for numerous city purposes. It consists of three numbers, separated by slashes: the borough, which is 1 digit; the block number, which is up to 5 digits; and the lot number, which is up to 4 digits.

The borough number is:

  1. Manhattan (New York County)
  2. Bronx (Bronx County)
  3. Brooklyn (Kings County)
  4. Queens (Queens County)
  5. Staten Island (Richmond County)

An example of a valid style of BBL is 4/99999/99 for Queens.
-- Wikipedia