Here are the typical places schools will "cut" the applicant pool down (marked with "X") along with an estimate of the percentage of people who usually make it far (note that this varies wildly between schools and is not representative of the general process):
Primary Applicant (100%) X1 Secondary Applicant (90%) X2 Applicant pool (85%) X3 "Small" pool (40%) X4 Interview Invitation (15%) X5 Accepted (11%)
X1: Schools often screen the primary application based on statistics (MCAT and GPA are the most common)
X2: Secondaries usually ask for information regarding past academic or legal offenses, and these MAY screen you out (though often they do not)
X3: Once your primary and secondary are submitted, some schools will "small-pool" you. This is just a (much) smaller group of applicants (see estimates) that you need to make it into in order to get an interview invitation. Some schools do not small-pool any of their applicants, and thus this step is nonexistent.
X4: From the small-pool, the adcom may still decide not to interview you. As you can see by my estimates, a large chunk of the small pool is still not interviewed.
X5: Once you're interviewed, they may waitlist or deny you, but the majority of interviewees are eventually accepted.
I know this may be more than you were asking for, but hopefully it gives you a good idea of the general process and how "small-pooling" fits into things.