Crash Course on Menzies (Building 11):
The letter represents whether it's West, East or South wing.
If there are three numbers, then the first number represents the floor number, and the following numbers represent the room number.
If there are four numbers, then the first *two* numbers represent the floor number, and the following numbers represent the room number.
If there is a B, then this represents the basement floor (read: go downstairs instead of up the escalator); the numbers represent the room number.
If there is a H followed by one or two numbers, then that's a lecture theatre... you don't actually go into the Menzies main entrance to get to these; you walk around outside the building to reach them.
Examples:
E143 - East wing, 1st floor, room 43.
W731 - West wing, 7th floor, room 31.
S1002 - South wing, 10th floor, room 2.
WB10 - West wing, basement floor, room 10.
H4 - Lecture theatre; reached by walking around the outside of Menzies.