well when you pull an embryo off a mare that mare cannot breed herself during that month so she needs another one to carry it for her. And in my case I have stored embryos from mares that are now retired and can't give birth anymore. The embryo needs a "mom" so to speak, it's not like a pokeball where you throw it to the ground and a foal pops out
it means she had an embryo implanted this year, probably from a much higher rated mare and therefore cannot have a foal on her own. it will reset on the 1st of October