When you choose "no substitutions," that has to do with what day you are originally scheduled to receive your birds. Our website is updated automatically every five minutes, so it is as accurate as possible. If you try to order the very last bird/s of a breed when someone else has already done so within those five minutes, though, the bird you ordered will not be available, and we'll want to know if you'd rather keep the date you requested, or if you would rather have a substitute breed. We'll want to know what substitutes you would be willing to accept, as well.
"Hatch day substitutions" are different. This choice has to do with letting us know how you want us to handle it if there are unexpected issues on the day your babies are set to hatch, but AFTER you have already been scheduled for your ship day. Depending on the size and composition of your order, choosing "no hatch day substitutions" can mean that if there is the rare problem on hatch day, your order will be postponed and shipped complete on another week... OR it can mean that your order will be shipped short. Those are the two possibilities, and what happens is determined on a case-by-case basis. For instance, if you order 18 birds, and two aren't available on hatch day because fewer females of that breed hatched than we expected, then your order might be sent with only 16 (rather than 18).
But if shipping short would put your order below your safe minimum for your location, then obviously we can't ship your order short. In that case, your order would have to be postponed until the next available date instead, so long as it is not one of your blackout dates. And remember, the next available date isn't always the very next week; sometimes a new ship date can be several or more weeks away, especially with very rare breeds or with combinations of many breeds in a single order. This is why we don't like to postpone orders if possible, and why we are so conservative when estimating what we expect to hatch. Remember, we are literally in the business of counting our chickens before they hatch, which is a practice that traditional wisdom does NOT recommend.
Nonetheless, we try to deal with these rare problems on a case-by-case basis, based on what you told us your preferences were when you placed your order. For that reason, be sure you think through and let us know exactly what you would like to happen in the case of a problem. Issues are quite rare, but you must let us know what to do "just in case," because you don't want to have your order delayed if you have arranged for your grandchildren to visit from across the country to see them! And you don't want to receive a substitution if you have anticipated for months getting your favorite rare breed--the one breed you really wanted, and the one that led you to place your order in the first place!
Whether to choose substitutions or not will just depend on your personal needs, so be sure to let us know what you want to happen in case Mother Nature pitches a curve ball!