The operator looks at the build sheet in the appropriate box and sees he needs a "TAF".
He then walks over to this display of various racks, sorted by engine type, chooses a "TAF", and hangs it on the line.
Another reason for the broadcast code (also known as a "pick code") is that the part numbers themselves are so nuanced.
Can you imagine if he really wanted a part number 12345678 but incorrectly picked a 12345
478?
It could be catastrophic.
K