[Users] Thin Provisioning

Hello everybody, I'm following the Quick Start Guide to clone a machine from a Template that it's already created. As thi guide says it's necessary to select in Allocation tab from Resource Allocation, Thin Provision. When I try to do this and down the select field it doesn't appear this option, I only can see Preallocated. I have oVirt engine 3.1. Can somebody give some light why I haven't this option in my webadmin? Many thanks in avanced, Juanjo.