Waist
The measurements of a model are important for the clients to see if the model can fit their clothes. So, one of the three measurements which you should know about yourself is the waist.
It will always be measured by your agencies. You will wear a bikini and they measure the smallest part of your torso, where your body dips in. (of course you can also do it by yourself)
The other two important measurements are the bust and the hips.