A single product can sit in several Shopify collections at once - a jacket that’s in Outerwear, New arrivals and a seasonal Sale collection, all at the same time. Only one of those can appear in the breadcrumb on the product page, so Breadcrumbs Uncomplicated has to choose.
When the web address already answers the question
If a shopper arrives on a long-form product web address, the kind that includes a collection in the path (/collections/outerwear/products/jacket-name), the choice is already made: the collection in that web address is the one the breadcrumb follows. This is the common case when a shopper clicks through from a collection page - the link they clicked already carries the collection they were browsing.
When it doesn’t - the short-form web address
A shopper can also land on the short-form web address (/products/jacket-name), with no collection in the path at all - from Shopify search results, an email link, a QR code, or anywhere else the collection context is missing. Here Breadcrumbs Uncomplicated picks automatically from the collections the product belongs to, using the Breadcrumb selection on product pages setting in the Theme Editor.
The default is Most specific (smallest collection) breadcrumb: among the collections a product is in that are part of your store’s category tree (the ones linked somewhere in your Navigation menu), the app follows the one with the fewest products, falling back to the product’s first collection if none of them are in the tree. The app picks by product count, not by position in your menu - a collection with fewer products is treated as more specific, and that’s usually the leaf category in your tree: the jacket above resolves to Outerwear rather than the much larger Sale collection.
For this to work well, make sure the collection you want as the trail’s leaf is correctly nested under its parent collections in your store’s Navigation menu - the app builds the rest of the trail from that menu structure automatically. The product itself only needs to belong to that one, most specific collection; adding it to the parent collections as well has no effect on the breadcrumb trail.
Choosing a different rule
Most specific (smallest collection) breadcrumb is the setting most stores want, but it isn’t the only option. Open the Theme Editor, select the Category Breadcrumbs block on a product template, and the Breadcrumb selection on product pages dropdown offers Most general (largest collection) breadcrumb, Show first possible breadcrumb, Show last possible breadcrumb, or Show generic breadcrumb as alternatives, for stores whose category structure works better with a different rule.
Overriding a single product
Sometimes the automatic pick isn’t right for one particular product, whatever rule the store-wide setting uses. Setting the Default Collection Reference metafield (custom.default_collection_reference) on that product tells Breadcrumbs Uncomplicated exactly which collection to use for it, overriding the automatic rule for that product alone. Every other product keeps following the store-wide setting.
Summary
A product in several collections shows one breadcrumb at a time: the collection in the web address when there is one, and an automatic pick, most specific by default, when there isn’t. The Breadcrumb selection on product pages setting changes the rule everywhere the block is placed (in practice, the whole store for most setups), and the Default Collection Reference metafield changes it for one product at a time.