I wanted to provide some more information about how secondary (also called supplemental) applications work.
Secondary applications tend to cost more than the primary/AMCAS application. (Yes, I have to pay application fees twice.) They are typically comprised of a reiteration of school information (found in your AMCAS application) and short answer essays, usually pertaining to the school. They often ask about how you will contribute to the diversity of the incoming class, or how X-school will help you reach your future goals, et cetera.
Most private medical schools (including the ones in Day 501) send secondary applications to all applicants.
Most public/in-state medical schools send secondary applications to screened applicants. "Screened" means they've seen your GPA, MCAT score, personal statement and all the other basic information provided in your primary/AMCAS application and they like what they see. This is definitely a good sign. You've jumped through the first hoop and you only have a few more to go
Just so you know, I've filled out all the private school secondaries I'm going to fill out. There are some I have received but have decided not to complete. This may change in the upcoming months depending on how my other secondaries fare.