A "landrace" is not a standardized variety, but rather a diverse population with similar characteristics. The term is generally used to describe localized populations developed through traditional farming, which means through on-farm selection by farmers. Most seed companies don't sell landraces because customers generally want to know what they're going to get. We love planting landraces precisely because we don't know what we're going to get. Many old landraces are under threat of extinction or have already disappeared. The loss of landraces is a huge threat to the food supply, because not only do landraces provide critical genetic diversity for future crop improvement, but also their diversity makes them incredibly resilient in the face of the extreme weather and explosion of pests brought about by climate change.
By "breeding stock" we mean varieties and populations with particular utility for plant breeders. This certainly doesn't mean the rest of the seeds we sell are not useful for plant breeders, but this category contains plants with certain unique and important traits (like the high anthocyanin "blue" tomatoes, or the modern disease resistance of 'Legend' and 'Rutgers 250 Schermerhorn' tomatoes). It also contains populations with a wide degree of genetic diversity like a landrace, but which don't exactly fit the definition (like Chris Homanics' perennial kale, Andy Hahn's hablitzia, or our own rhubarb breeders mix).
EFN is all about preserving and expanding agrobiodiversity. This category is for growers interested in using their garden or farm to help further this important mission.