The search screen is a window into the actual North Texas MLS that agents use daily.
All agents search by "areas" along with a combination of subdivisions, price, etc. to locate properties to show clients.
Here is a picture of the search page that we have marked up to help get you started.

Agents work with geographical areas based on an Area Map that the general public isn't normally exposed to. For instance, Mansfield is area 89.
Here we have provided a map of the Mansfield/Arlington/Grand Prairie/Kennedale areas. Your best bet is to print this map out.