what is the program entry label _iar_program_start use for in IAR Workbench for ARM?

I am developping a firmware on an ARM CORTEX M3 processor, with IAR IDE (v6.50). I am wondering the usefullness of the label used for the program entry. I explain.

On the ARM platform, when the chip is reset the PC is loaded with one of the entry of the vector table (the second entry in my case). So the program will start from this adress at power time and at every next reset. So the start address depends on what is declared in this entry. But in the IAR options, it is possible to define a label as the program entry. By default, the program entry is __iar_program_start. It is possible to override it.

But if the PC is loaded with the second entry of the vector table on reset, what is the IAR entry point used for?

I am wondering that question because I am trying to integrate the CMSIS layer in my existing firmware, and the entry point is not __iar_program_start anymore, but Reset_Handler. In the IAR option, I first tryed to override the default entry point __iar_program_start by "Reset_Handler" but I had an ielftool error ("ielftool error: The string '__vector_table' was not found in the string table "). The solution is to let the default program entry point in the IAR linker option but I can't understand why it souldn't be the Reset_Handler label.

Here's a part of my startup code for the example (it's running):

        MODULE  ?cstartup

        ;; Forward declaration of sections.
        SECTION CSTACK:DATA:NOROOT(3)
        SECTION PSTACK:DATA:NOROOT(3)

        SECTION .intvec:CODE:NOROOT(2)

        EXTERN  __cmain     
        EXTERN  SystemInit
        EXTERN  vPortSVCHandler                                 ; SVCall_Handler is renamed here
        EXTERN  xPortPendSVHandler                              ; PendsSV_Handler is renamed here
        EXTERN  xPortSysTickHandler                             ; SysTick_Handler is renamed here

        PUBLIC  __vector_table
        PUBLIC  __vector_table_0x1c
        PUBLIC  __Vectors
        PUBLIC  __Vectors_End
        PUBLIC  __Vectors_Size

        DATA

    __vector_table:
        DCD     sfe(CSTACK)                 ; Top of Stack
        DCD     Reset_Handler               ; Reset Handler

        DCD     NMI_Handler                 ; NMI Handler
        DCD     HardFault_Handler           ; Hard Fault Handler
        DCD     MemManage_Handler           ; MPU Fault Handler
        DCD     BusFault_Handler            ; Bus Fault Handler
        DCD     UsageFault_Handler          ; Usage Fault Handler
    __vector_table_0x1c:
            DCD     0                           ; Reserved
            DCD     0                           ; Reserved
            DCD     0                           ; Reserved
            DCD     0                           ; Reserved
            DCD     vPortSVCHandler             ; SVCall Handler
            DCD     DebugMon_Handler            ; Debug Monitor Handler
            DCD     0                           ; Reserved
            DCD     xPortPendSVHandler          ; PendSV Handler
            DCD     xPortSysTickHandler         ; SysTick Handler

        ; External Interrupts
        ...

__Vectors_End

__Vectors       EQU   __vector_table
__Vectors_Size  EQU   __Vectors_End - __Vectors

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;
;; Default interrupt handlers.
;;

       THUMB
       PUBWEAK Reset_Handler
       SECTION .text:CODE:REORDER(2)

Reset_Handler
        LDR     R0, =SystemInit
        BLX     R0
        LDR     R0, =__iar_program_start
        BX      R0

        ... Other handlers....

__iar_program_start:    
        ;Process stack initialization 
        MRS     r0, CONTROL       ; Get PSR
        ORR     r0,r0,#0x00000002 ; Use Process Stack instead of Main Stack
        MSR     CONTROL, r0       ; 
        ISB                       ; flush pipeline
        LDR     sp, =SFE(PSTACK)  ; init Process Stack address

        ;Get back with Main Stack
        MRS     r0, CONTROL
        AND     r0,r0,#0xFFFFFFFD
        MSR     CONTROL, r0
        ISB

        BL  __cmain

        END

This is old but I'll answer it anyways.

If you look at startup_.s you should see something like this

Reset_Handler
    CPSID   I               ; Mask interrupts
    LDR     R0, =0xE000ED08
    LDR     R1, =__vector_table
    STR     R1, [R0]
    LDR     R2, [R1]
    MSR     MSP, R2
    LDR     R0, =SystemInit
    BLX     R0
    CPSIE   I               ; Unmask interrupts
    LDR     R0, =__iar_program_start
    BX      R0

As you can see Reset_Handler branches to __iar_program_start