The Inventory is a base feature in SFH 3 where all player's items are stored. It can have maximum 264 items.
Items get into the inventory after you buy them in Store, win them on the Slot Machine or win them after battles. While equipping hero with weapons or mods, you can acess 6 sections: 4 for weapon types and 2 for equipment and weapon attachments. However, Elite and Engineer have access to 7, because they can use all 3 secondary weapon types unlike other classes.
If you win a weapon by Slot Machine or after battles but your inventory section is full, don't worry, you CAN access inventory and sell items you want at any time.