oh... the bullet with the devour icon. it can be grey or red colour.
the slot is originally empty unless you devour the enemy before the enemy dies.
instead of devouring the enemy after they die (for EXP), u do it before they die, u get some yellow glowing stuff on ur character and a bar at the bottom center of the screen. with some Lvl 1 stuff and it gets orange at lvl 2, havnt reach lvl 3 yet, so i'm not sure what lvl 3 does.
even after the bar expires, the bullet stays there. its either u get 3 or 4 charges for that special bullet.
Thank you for answering. For you infomation, they are called Abyss Aragami Bullets (AA Bullets). I see you have done a good job on GEO page and I hope you will keep up with your contribution when the game is released^^
thank you for enlightening me... actually i don't know much about God Eater except the anime and GEO. There are a lot of terms i need to learn from this wiki... i hope i have more time, since im managing FGO wiki