What is "secondary defence"?

I know players will chuckle when they see I don't even know what it means; but I dont;
is secondary defence the militia or conscripts or both???

"Secondary defense" is an unit role. in atWar, units can have a main role (Ground, Naval, Air) and a secondary role (Main Attack, Main defense, Secondary defense, other, etc). If you get to play any custom map, be sure to check the unit's role in order to avoid any confusion.

Militias are the default map's Ground: Secondary Defense units. In some custom maps conscrips can be either Main Defense or Secondary Defense, you must check in the units menu.

Thanks Clovis!