As you can see from the example here, there are three (3) components to the drawing number structure.
The first two (2) letters are called the discipline designator indicating the discipline/design profession or trade.
The third (3rd) digit represents the sheet type of drawing (i.e. plan views, details, sections, schedules etc).
Finally, the last two (2) digits are basically the sequence number of drawings in the series of the same type. They can be any sequential number from 00 to 99 and the purpose is to place the drawings in order.