Areas not under the control of any nation are neutral.
There is no need to specify this in the xml files, any units or areas not assigned to a specific nation are neutral.
There must be neutral flags and all required unit art in the games map folder plus a color set in the map.properties file.
Though there are game properties options for neutrals, if you want the neutrals to do specific things it is best to make AI_Players.
Some games neutral rules:
| NGE | Spend 3 to invade neutral. Some neutrals give income to specific nations while neutral |
AAC neutrals are invadable at cost of 3, but have no income. In other official versions, they are impassable. In many TripleA scenarios, neutrals are just areas like any other, with fixed garrisons and income.