The purchasing managers, category buyers, sales marketing team all have different inputs to what assortment of products are cataloged for sale. The setup of an item encapsulates the business rules that the purchasing, sales, accounting, inventory teams want to specify to control the item.
This video walks through some of those basic concepts of what an item is in Dynamics AX, how to setup a basic item, some of the parameters on the released item. What I do is also walk through a quick purchase and sale so you can see the basic effect in the systems.
Cheers
Lachlan