I'd like help completing my answer (below).
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In OpenERP, product categories are used when accounting transactions related to products need to use more than one account.
You would use them when you want or need:
different income and expense accounts for different products
different stock valuation accounts for different products
different price difference accounts (differences between the invoice price and standard price - also called purchase price variance)
Note that (3) is only applicable if you have the Anglo Saxon accounting module installed.
It is also possible to categorize products to make them easier to find, because you can filter by category. Because a product can only belong to one category, it may or may not be useful to use product categories for this purpose.
There are other modules that support products belonging to more than one category (for better integration with e-commerce systems such as Magento for example) but they require you to pick one category to be the 'master' category for each product.
To view, edit and create product categories :-
Sales â Configuration â Products â Products Categories
(and also directly from the Product form view)
To view a hierarchical list of all product categories :-
Sales â Products â Products by Category
I have also had a lot of reflection about this issue. A big help for not having to enter too much double information is Ray's comment in front of mine, what I also discovered some days ago: To base purchase price calculation on product price depending on supplier, it is enough to insert product category to the pricelist. If product categories are properly chosen, pricelist does not have to be touched very often when adding a new product. I am using product categories to assign right accounts to every product.
So far, so good. My case a bit special: It can also occur that the same product needs to have assigned more than one accounts set. This depends on what supplier it is bought from. There are some who no purchase taxes need to be paid with. Therefore, I have to hold such products redundantly only differing in product category. Any idea if there is any way to avoid this? I have hoped to be able to connect this accounting information to the supplier, but it does not seem that this is possible.