Depending on each World's congestion, there may be Worlds that do not separate areas into Field Instances. Please check the "Worlds Currently Using Field Instances" section to see which Worlds are currently utilizing Field Instances.

Those worlds are automatically going to start with the instances and other worlds will use them as the need arises. I imagine they have an instance cap which will automatically trigger the initialization of the field instances.

For example

Chocobo / Shinryu / Bahamut / Carbuncle / Balmung / Gilgamesh / Excalibur / Hyperion

OMG WE ALREADY KNOW WE ARE JUST GOING TO LEAVE ALL THREE ON.


Everybody else

Its set to trigger Field instances as each instance fills up.

Two games that use this on the top of my mind are Elder Scrolls online and Star Wars the Old Republic.