Terrain 'zero' is a technical type of terrain used to create maps.
You can use it to draw a graticule or whatever you need.
However you can not load scenario with this type of terrain on the map, when playing the game.
'Delete zero' will change all zero terrain type to plains and allow you to run the map by game.
'Allow invasion' sets where amph landing can be done.
'Show invasion' colors all hexes where amph landing can be done into white.