ES does not measure its license consumption.
ES itself doesn't have it's own license consumption model.
Licensing depends on the DM's you are using and which indexes they are including. Total volume used by ES is the total volume of indexes used in it.
Ok, but if the index used by the DM is only partially used, let's say, only 10% of the index is needed, how does it work?
Yeah that can be an issue especially since most of the time you end up paying an ES license equal to your core license because it's assumed that "most of the volume will end up in ES anyway".
Best way to handle this problem is to talk to the sales rep and see at what point your "license" will cost you more. As it is now, the only thing the license does it allow you to get the ES installer and install it, doesn't limit your use of the application.