How does building selection work?

Hi! I just received an email about my building selection time and I’m just a little confused on how the whole process works. Does the person with the earlier time have to reserve the room for their roommate or is there anything special about the process. Just feel kind of in the dark so if any one could help me clear things up and explain that’d be awesome. For reference my selection date for Summer is May 4 at 10 am and for Fall is May 18 at 10:30 am.