I want to know how will my game be marked as adult content, and as far as i know having your game marked as adult content will limit it's visibility. having Sex screenshot or porn will mark the game as adult content(NSFW) but what about other content like high level of blood or Gore.

to make it simple I'm working on a FPS horror game and my game contain to much Blood and  uses a sophisticated gore system. A powerful finishing blow can rip a part of the enemy right out of him, while critically over damaging an opponent literally blows them to pieces, with limbs flying across the room and sticking to the walls.

thanks for help.

You have to mark your game yourself. If you let staff do it, chances are you will be banned or delisted, as not marking it, if you were required to, is a tos violation.

If you are in doubt, look at any similar game that is on Steam. They write age restrictions there. Just guess what your game would get as a classification.

That being said, what constitutes nsfw is quite arbitrary and inconsistent in translation to an age rating and vice versa. This goes as well for the wording used in the faq "strong adult themes". It is debateable what this is or where the line is. And to make it complicated, worldwide standards are different. Itch is us based and so is the nsfw term. Boobs will get you far quicker in that territory, than pixel blood. 

But unless you do a comedy zombie game like plants vs zombies, chances are your game is not aimed at teenagers, so why do you even need to ask? Just mark your gore game.