Why are there different vacation accrual rates?

Generally, vacation accrual rates are based on the employee’s length of service with the University. There are also some differences depending on the employee’s personnel program. Refer to the PPSM Absence from Work Policy  (PDF) and labor contracts for specific accrual rates.

What does it mean to record time to the nearest quarter hour?

Time is reported to the nearest quarter hour only for employees in non-exempt positions. When a non-exempt employee takes time off for sick leave and vacation leave (if it is less than a full day), it is recorded to the nearest 15-minute increment.
For example, if an employee returns from a medical appointment at 17 minutes past the hour, you would record 15 minutes. If the employee returns at 24 minutes past the hour, you would record 30 minutes.

What is the difference between exempt and non-exempt positions for time-keeping purposes?

Employees in non-exempt positions accrue vacation leave and sick leave depending on percentage of appointment and time worked, and/or duration of appointment. They can earn overtime and may be paid for it or take compensatory time. Time off and time worked are recorded to the nearest quarter hour. Note: Part-time non-exempt fixed-percentage employees must always meet their percentage of time for hours worked for each week, as well as the entire month (excluding any holidays).
Employees in exempt positions also accrue vacation and sick leave; however, they do not earn overtime or compensatory time. Time off and time worked are recorded in whole-day increments for purposes of pay. A "whole day" may be less than eight hours if an employee’s appointment is less than 100% time.