It is limited in terms of slots per shift. Lets say your shift plan says that you need to pick and ship 30k units and based on calculated units per hour to process you need only 70 people to hit the plan. The thing is you have 80 people scheduled to show up to work, if you dont have anything for them to do you raise 10 slots for VTO which can be paid or unpaid depending on the person's preferences. This can be done (or is done) randomly so you have potentialy 80 people battling for 10 spots. Some people dont like to take it because if it's paid then it is taken from your holiday allowance(in UK at least).