The pending option is for time that an employee can accrue but is not available to take by the employee yet.  For example, the first 90 days of employment may be probationary.  An employee can accrue their time; however it is not available to them until they complete the 90 day probation.  In this scenario, we would use a One Time waiting period and 90 days in the Leave Setup window.  

When the system calculates leave time during the first 90 days, the time appears in the pending field until it is available after the employee completes the 90-day probation.  In our setup the Accrual Based on date equals the hire date thus the 90-days is 90 days after the hire date.  On the Employee Leave Maintenance (Cards > Payroll > Leave) window you can see the system defaults the Accrual Based On Date as the Waiting Period Start Date and we have entered 90 days as the waiting period units.

CLM Pending 1

If you mark the option to Calculate Earned Time during Payroll Pro