What breadcrumb markup does
It shows Google where a page sits in your site: Home › Blog › This article. Google uses it to understand your site's structure and, on desktop, shows the trail in place of the page's address. On mobile, Google now shows just the domain.
Each level needs a name and a position. Every level except the last also needs its address.
Match what people see
The trail in the markup should match the breadcrumb links shown on the page, if you have them. If a page can be reached in several ways, pick the path that best fits how people browse.