There has been discussion around this question, but my current thinking is that it is “seeds that cannot be conserved long-term in conventional seed banks.” This includes species without seeds (either because they are not producing seeds, or sufficient seeds, or accessible seeds), recalcitrant seeds, cold sensitive seeds, and short-lived seeds. Of course the terms short-lived and long-lived are not precise, although one definition has been those species with longevities <20% of the maximum longevity of seeds. I don’t think that is published at this point. Other definitions I’ve heard are 10 or 20 years longevity at -20oC.