The Common App exists to remove repetition. Before it, each university required its own form with much of the same information. Now the shared part is entered once.
What the system solves
Personal details, education history, activities, and the main essay are entered once and sent to every institution on the list. This is the shared section.
It does not make applications interchangeable, because of what sits beside it.
The shared section and the supplements
Most selective universities add supplementary questions of their own. These are short essays, usually asking why this institution and why this programme.
The supplements are where applications are differentiated, and they are also where students run out of time, because a list of ten institutions can mean twenty short essays on top of the main one.
What the school sends
Parts of the file do not come from the student. The counsellor submits a school report and a school profile, and teachers submit their letters directly.
These pieces have their own timing and depend on other people. Requesting them late is the most common reason a file is incomplete at the deadline, and it is not something the student can fix alone at that point.
Fees and waivers
A fee is charged per institution, which is why the size of the list has a direct cost. Fee waivers exist for students who meet published criteria, and the waiver is requested inside the application rather than separately.
The calendar
The application opens in the summer preceding the application year. Early deadlines fall in the autumn and regular deadlines in the winter, and each institution publishes its own.
Because the platform does not enforce a single date, a student with a mixed list is managing several deadlines at once.
The most common mistake
Writing a single supplementary essay and adapting it by changing the institution's name. Admissions readers see many of these, and a generic answer to a question about fit reads as an answer about nothing.