Standard-Professional-Premium?!

Bonjour!
I've used for many years the free versione of DraftSight and now I'm going to buy a license. I think that the Free license was like the Professional one, but I'm trying to understand the difference between the three version because I probabillly need to buy two license: one for everyday use and one for a secondary pc where should be enought a Standard license for ocasional use.
If the Professional one is like the Free one, what is missing in the Standar one compared to the Professional? And about the interfaces, are they equal?