Ownable lots are introduced by World Adventures to differentiate potential vacation homes from the local townies' homes. In order to see the lot type, you need a mod that adresses travel and/or lot types, like the one mentioned above.
The difference between an Ownable and Regular residential lot is that the active family cannot buy a Regular lot in a vacation world, and Ownable lots have a special reservation to leave them available for player purchase (the level of exclusivity may depend on whose Story Progression you're running).
Once owned, there is no noticable difference between the lot types. The process to move to them, build/buy and sell them is identical. Under "Check Real Estate," both types will appear as Private Lots if they're on your homeworld and Vacation Homes if they're not.