Reserve capacity provides functionality allowing a department to control class composition. With reserve capacity, a number of seats can be set aside (reserved) for specific student populations, and only students in those groups can fill those seats. Later, during the enrollment period, any unused seats can be made available to the rest of the student body. For example, in a class of 20 people, the faculty can specify that 10 students have to be from a particular major/minor, and if by a specific date only 6 majors/minors have enrolled the remaining 4 reserved seats will be able to be filled by non-majors/minors.
As a student if you do not meet the conditions of the reserve capacity (see screenshot below) you still have the option of resubmitting your enrollment request and choosing the waitlist option. If the reserved seats are not filled at the end of the reserve period the waitlist process will automatically place you into the course provided the overall course capacity has not been reached.