You currently attribute product purchases to a project, that is, you would only impute products that you purchased at the time of the project. In certain businesses you stock up on certain products, for example a gardener will buy kg of fertilizer because he knows that he will have projects in the future where he can use that fertilizer. When it comes to charging this purchase as a user, you must search for that old purchase and distribute the units among the different projects. It would be interesting to have a faster option to attribute the product to the cost/expense of the project, without looking for such a purchase.