Basicly it's a tool that you can use to make an object only appear when you are playing a Symmetrical gametype on your map, Like Team slayer ,Multi flag or any other gametypes when you have 2 teams.
This is an option where you can set objects to spawn in symmetrical games, asymmetrical games, or both. Asymmetrical games are games like one bomb, one flag, territories, and pretty much anything where you have one team only attacking and one team only defending. Symmetrical games are the opposite, like Team Slayer, Multi Flag, Assault, and VIP. So say in one bomb you want one team to have a turret but the other team not to have one. And if you ddidn't know about Symmetry in every game that team would have an advantage. So you place the turret press "x" and go to both. Since you want that turret for one bomb set it to aysmmetrical and then in Assault that turret will not be there. However in one bomb it will be. I hope that helps