When the option appears

Brand is offered only if the product_brand taxonomy exists on your site, which it does when WooCommerce’s brands are in use. On a shop without them, the option is not in the list, and a set that was built elsewhere with a brand filter simply leaves it out instead of failing.

What it filters

The brand filter works like the category and tag filters: Siftwren, not WooCommerce, applies the choice, because WooCommerce has no address parameter for it. It does so on the shop’s main query and nowhere else, and it falls back to doing nothing if anything about it fails. The numbers beside each brand are worked out against everything else you have chosen.

Brand pages

A set is drawn on product brand pages too, along with the shop and the category and tag pages. On a brand page, the brand’s own products are the starting point, and the other filters narrow them. If you place the same set in the shop and on brand pages, you get the same controls everywhere.

Setting it up

Add a filter and choose Filter on: Brand. Checkboxes suit a handful of brands, and a dropdown copes better with fifty. An address carries at most fifty values per filter, which no shopper will meet in practice.

If your brands are product attributes instead, such as a global attribute called Brand, use Attribute: Brand and the parameters in the attribute guide.