urgent need to know about STM32F207VC interrupts

hi:

im bit new to stm32f2 micrcontroller a problem in configuring the NVIC IQRchannels.. why it is the two configurations for preempriority and subpriority each as the priority saving register contains whole for both preemtion and subpriority both.

==========================================================================================================================
* NVIC_PriorityGroup | NVIC_IRQChannelPreemptionPriority | NVIC_IRQChannelSubPriority | Description
* ==========================================================================================================================
* NVIC_PriorityGroup_0 | 0 | 0-15 | 0 bits for pre-emption priority
* | | | 4 bits for subpriority
* --------------------------------------------------------------------------------------------------------------------------
* NVIC_PriorityGroup_1 | 0-1 | 0-7 | 1 bits for pre-emption priority
* | | | 3 bits for subpriority
* --------------------------------------------------------------------------------------------------------------------------
* NVIC_PriorityGroup_2 | 0-3 | 0-3 | 2 bits for pre-emption priority
* | | | 2 bits for subpriority
* --------------------------------------------------------------------------------------------------------------------------
* NVIC_PriorityGroup_3 | 0-7 | 0-1 | 3 bits for pre-emption priority
* | | | 1 bits for subpriority
* --------------------------------------------------------------------------------------------------------------------------
* NVIC_PriorityGroup_4 | 0-15 | 0 | 4 bits for pre-emption priority
* | | | 0 bits for subpriority
* ==========================================================================================================================

and also i cant undersatnd what this tables says...

plz im in need of help... thank you.