One of the things I wanted to avoid when designing Seref’s inventory system was the common weight based system: that is, a system where you can carry anything and everything so long as you’re strong enough to carry the weight. This has never made particularly much sense to me, and I wanted something a bit more logical, so we went with a system based on storage space and the size of items.
Seref’s inventory system uses a inventory slots, which represent how much space any given container has, in order to determine just how much a character can carry. The standard backpack has twenty inventory slots in its main pouch, and carry lots of little items, or only one or two large items; exactly like an actual backpack. Some items won’t be able to fit in certain containers: for example, a spear won’t fit in a backpack, even if a spear takes up less than twenty inventory slots. For the most part, this applies to larger items like weapons and more rigid armour. What your character has with them becomes a matter of what they have space for, rather than what they can lift, with various options for increasing space, such as through pouches and pack horses.