I think what they are asking in a common problem encountered by ecommerce user selling by weight.
Let say you sell powder, any kind ;o)
you have in stock 10 000 grams = 10 kg
you sell that product in different quantities
10 gr bags
50 gr bags
100 gr bags
500 gr bags
1 kg bags
This means when a customers buys 5x10gr and 2x100gr bags your stocks decrease from 250 gr so this leaves you with 9 750 grams in stock.
But usually this type of ecommerce sells also products by unit or ml.
This method would help keeping acurate stock without having to create one article for each item and missing sale when one is our of stock, as it is easy when you are out of 10gr bags to use a 500gr bags left and divide it 50x10gr bags, also if there is only 50gr left customers are not able to buy 100gr or more quantities.
Personally I never was able to make this work on OSCommerce, just start to touch to attributes can give headaches. May be Magento will be able to do this. Did not try Magento yet, but it looks very professional.
Patrick from France
PS: - I would love to change the world, but they won’t give me the source code - !!!!!!!!! I wish I could Help :o( !!!!!!!!!!!!!!!!!!!!