The tractor will have two belts one goes from the engine to the transmission and one from the engine to the deck. If it is the one going to the deck - deck belt.
There is another belt up under the tractor that drives the wheels (drive belt).
The L100 you could get the deck belts in two different size decks.
For diagram of the pulley or extra info please visit this link
The manual will cover it.
How to fix it
How to install a mower deck belt