I assume you mean the markings on currency that indicates it's Federal Branch origin.
If so, here are the letter and number designations
Boston - A 1
New York City - B 2
Philadelphia - C 3
Cleveland - D 4
Richmond - E 5
Atlanta - F 6
Chicago - G 7
St. Louis - H 8
Minneapolis - I 9
Kansas City, MO - J 10
Dallas - K 11
San Francisco - L 12
On older bills, the letter appears in a seal on the left side of the bill and the number appears on the left.
On newer bills, the letter and number appear on the left below the serial number.
