Unused Land
Unused land is a type of land that can be found both on mainland and estate land. It is almost impossible to estimate the amount of unused land that currently exists. From a point of view, the Abandoned Land can be considered unused land.
Status
Unused land can belong to a resident or a group (usually to a land baron) or can be Linden-owned. Almost always when it is private-owned, it is for sale or for rent. To find out its status, walk on that land and choose World - About Land'. Usually, there is nothing built on that land except for a land tier box or an information box. A special type is a prim reserve. Some residents use only a parcel while another parcel is used to increase prim limit.
Land For Sale
Usually, land for sale is unused until a customer decides t obuy it and build something on it. More information on Land For Rent Or Sale.
Advantages
Unused land usually has no ban lines or entity orbs, so avatars and vehicles can travel on it. Also, it usually has no teleport restrictions, so it can be used as an entry point for visiting a place. Also, if you consider an unused parcel to be a good location for your own home, it will be much more easy to convince the owner of that parcel to sell it to you then if there is something built on it.
Linden owned
Not to be confused with protected land, that is a public land like a park or a transportation way.
Unusold land. This type of land is rare. Unsold parcels can be found in Gaeta 1. It is formed of many sims where land was never sold to a resident.
Reserve land. This type of land can be found in the Premium Continents. It is a reserve for extra premium homes. Small percels of reserve land can be found in any sim with Linden homes. In Premium East continent, there are entire sims with reserve land.
Private land
There are two major forms of private unused land.
Estate land. In many private-owned sims, unused land can be found as land for sale or for rent. Usually, it is very easy to purchase land from this source.
Rental land. Many rental corporations have land on all mainland continents, from small parcels to entire sims. It is difficult to estimate how much of this land is used and how much is unused. Many times, constructions are not on ground, but on skyboxes. However, the largest amount of rental land can be found in Corsica, followed by Satori and East Sansara.
Prim land is a parcel used to increase prim limits to another parcel in the same sim.
Others. Sometimes, a parcel is unused and its owner never used its account for a long amount of time. Since premium tier can be paid for a whole year, even if the owner died, the land is paid and will remain to its current owner until no money is received for it.
Confusions
Many times, a parcel can be unused because its owner both it and had not time to build something on it. Also, constructions might be in a skybox.