Subareas are inside a land. These areas are three dimensional. That means that they are not bound to chunks, but instead to blocks. You can trust players to specific areas and apply flags to them, like you would do for a land. The land also has a default area which contains every block that isn't part of a subarea. This is the area that you're normally modifying in the menu of your land.

Create a new Area

1. First open your land menu with /lands menu and click on the areas item. Depending on your servers configuration the item positions may vary.
   

2. Now click on the add area button. After this you will need to enter the area name in chat.
   

3. After that you will notice that the area will popup in the menu and give you instructions on how to set select the boundaries of the area.
   

4. The selection is set. It will display it to you live. You can make changes as you wish. Then just execute /lands selection assign <area>. After that everything is setup. The GUI menu of the area can be opened via /lands or by standing inside it and executing /lands menu here.
   

Trust Players and set Flags

By opening the areas menu and clicking on the area, you can trust players to this specific area and edit flags. You can do everything for this specific area, just like for the default area.