in simplistic terms, the earlier the split occurs, the less in common the twins will have.. thats why all the way at the end of the line, you have an increased risk of conjoined twins since theyre sharing everything. but dizygotic twins occur when they split right away, so they each have their own chorion, amnion, placenta, etc.
Basically it's saying that if you have identical twins (monozygotic), the number of chorions/amniotic sacs depends on when the zygote splits. If it's in the first 3 days after fertilization, you get two chorions and two amniotic sacs (so they each get one of their own of both). If it's between days 3-8 they usually share a chorion but still have separate amniotic sacs. If it's after 8 days, they may share a chorion and an amniotic sac and there's a possibility they'll be conjoined twins. First aid states that this is the case because the chorion develops on day 3 and the amniotic sac on day 8.