Your inventory is an amount of goods and / or materials you own in your business. An item of inventory is a physical, tradable and / or usable item. The general purpose of an inventory item and obviously inventory total is always to make business.

Items of inventory are such that you use for either production, sales (that is you’re selling those items) or for your own use in running your business. The latter is something that’s rather not common, but happens nonetheless. 

Now production used materials are such that you can obviously also sell as they are, but you’d normally use them to generate something, an end product sort of say. Sellable goods are those that you buy from a supplier, a whole seller for an example and sell them forward to end customers (or, if applicable, to another seller).

Generally speaking items for your own use are those that you’d also sell for your customers, but every now and then you also need them for yourself and as such, you take them from your stock rather then going out and buying from someone else. They’re not something you intentionally keep for your own use because then they’d be directly debited into expenses.