This policy setting allows you to define the maximum time interval that the user can set for scheduling Exchange ActiveSync ...

"This policy setting allows you to define the maximum time interval that the user can set for scheduling Exchange ActiveSync synchronization during the off-peak service period. Peak is defined as the days and hours when wireless voice and data charges are highest. This policy allows you to control data costs.  If you enable this setting, then you can select the maximum time interval. The user can change the Exchange ActiveSync frequency schedule for the off-peak service period to any value that is less frequent than the policy setting.  If you disable or do not configure this policy setting, the user can define any off-peak Exchange ActiveSync frequency schedule in the Windows Mobile powered device."