The usage data is created without including the information of properties of the usage object. For example if we want to bill a vm with usagetype 1 we only see the time the vm was used. But we don't see in the usage data how many cpus, etc. the vm had at the time the usage record was created. Thus we cannot bill changes of the vm. For example if a customer adds a cpu core to his vm and later removes it, this should be seen in the usage records. But as far as I know the database records are all set to NULL and cloudmonkey does not return these fields although they are availble in the API.