The "item types" (B, M, D) are fixed in the code. When we create an item, we select one item type.

On the other hand, we can create "item categories" as we want. But we have to select an "item type" for the "item category". Why? The "item type" is already defined as in the first paragraph.

And further more, what happens if I create an item with an "item type" B and then I assign an "item category" with different "item type" as D for example? Confusing.

Thanks for an explanation.