The first number represents the fiscal year of the launch, the second represents the launch location (1=Florida, 2=California) and the alphanumeric code represents the order launched (i.e., A is first, B is second, etc.). Therefore, the first mission using this designation, which was the tenth shuttle mission was STS 41B. It launched in FY 1984, from Florida and was the first to launch in that year.
The system uses a fiscal year (first number), location (second number) and alphanumeric code (third number) to identify missions. The tenth shuttle mission, STS 41B, launched in FY 1984 from Florida and was the first launch of that year.