WooCommerce counts stock per product. Real stock is often shared: one roll of red fabric becomes shirts, scarves and bags; one batch of beans is sold in 250 g, 500 g and 1 kg packs; one box of blank mugs carries twenty designs. Shared Attribute Stock counts the thing you actually run out of.

One stock item, many products

Create a stock item, say "Red fabric", enter how much you have, and tick the attribute values that draw on it. Every product and variation with that value is now limited by that one figure. Limit it to certain categories if the same value means something else elsewhere.

Different amounts per sale

Say how much one sold item uses for each value: a small uses 1 and an extra-large 1.5, a single uses 1 and a pack of six uses 6, a 250 g bag uses 0.25. The shop shows how many of each product can still be made from what is left.

The whole cart is checked

Two different products that share a stock item cannot together take more than there is. The product page shows the number left, the quantity box stops at it, and "Out of stock" appears when it is gone.

Stock that adds up

Stock is taken when an order is placed and put back if it is cancelled, refunded or fails. Exactly what an order took is remembered on that order. Every movement is listed in the item's history with its order number, and you get an email when an item runs low.