It's a little more complicated. Members only send seeds to SSE if they request some. As a rule, members list their available seeds in SSE (both online, and in the annual print Yearbook) then take requests & ship their own seeds directly. SSE itself is not involved in those exchanges. Where it perhaps differs from SoDC is that SSE as a non-profit organization grows their own backups for many varieties, and maintains them in their seed bank -
if the variety meets their Accessions Policy. With limited exceptions, they only list their seed in the Exchange if no other member is offering that variety for the current year. Since many of those who originally donated to their seed bank no longer offer seeds, the number of varieties SSE offers as a lister has steadily increased (to 5059 listings this year).
It is admittedly a huge undertaking for SSE to grow & maintain 20,000+ varieties, only a percentage of which can be grown each year for renewal. To add to that, in an effort to ensure that seed being offered by them (and stored off-site) is true-to-type, a seed crop is often grown again for evaluation, before the seed is released for the Yearbook. That evaluation grow out is not necessarily in the year following the seed crop. I confess that system has occasionally been a source of frustration for me, when I observe a variety which interests me while visiting Heritage Farm, and am waiting for that seed to be offered. And waiting. And...



Regardless, I have only the highest respect for the preservation staff, which gets a difficult task done each year, and has often bent over backwards to be helpful.
SoDC does not, I believe, grow any seed as an organization. But as a member, do you send any seed in to them, to be held as backup? Is any seed offered by SoDC itself (perhaps for regeneration) or only my its members?