Land
Avatars have land associated with them, in the form of solar systems. Land is an important factor in City Abandonment and Decay.
The solar system that contains the avatar's home building is automatically part of their land. The solar system remains protected even if the home building is destroyed.
In addition to their home system, each avatar may designate up to ten solar systems to be their land. The land designation is arbitrary. Any solar systems can be chosen.
This does not claim those solar systems as property. There is no way to know if a solar system is already part of another avatar's land. Any number of avatars may designate the same solar system as their land.
Land is shown on a new Land page on the Bio window. There, land can be removed and the current solar system can be added to your land.
Land can also be added using the Cities and Bases, System Scan, and Star Map windows. The cities list supports multiple select, so you can do a bunch of them at once.
See the Abandonment and Decay pages for information on how Land links to them.
Update
This was introduced in the 2019-12-04 Update: http://hazeron.com/mybb/showthread.php?tid=2058