ContGen-IntClk - DoneCallback

I am following this example code, ContGen-IntClk.c.

#include <NIDAQmx.h>
#include <stdio.h>
#include <math.h>

#define     DAQmxErrChk (functionCall)      if( DAQmxFailed(error = (functionCall)) ) goto Error; else
#define     PI                              3.1415926535       
#define     DAQmxFailed(error)              ((error)<0)  

int32 CVICALLBACK DoneCallback (TaskHandle taskHandle, int32 status, void *callbackData);

int main(void)
{
    int32           error               = 0;
    TaskHandle      taskHandle          = 0;
    float64         data        [1000];
    char            errBuff     [2048]  = {'\0'};
    int             i                   = 0;

    for (; i < 1000; i++)       data [i] = 9.95 * sin( (double) i * 2.0 * PI / 1000.0);

    /*********************************************/
    // DAQmx Configure Code
    /*********************************************/
    DAQmxErrChk (DAQmxCreateTask            ("",&taskHandle)                                                    );
    DAQmxErrChk (DAQmxCreateAOVoltageChan   (taskHandle,"Dev1/ao0","",-10.0,10.0,DAQmx_Val_Volts,NULL)          );
    DAQmxErrChk (DAQmxCfgSampClkTiming      (taskHandle,"",1000.0,DAQmx_Val_Rising,DAQmx_Val_ContSamps,1000)    );
    DAQmxErrChk (DAQmxRegisterDoneEvent     (taskHandle,0,DoneCallback,NULL)                                    );

    /*********************************************/
    // DAQmx Write Code
    /*********************************************/
    DAQmxErrChk (DAQmxWriteAnalogF64        (taskHandle,1000,0,10.0,DAQmx_Val_GroupByChannel,data,NULL,NULL)    );

    /*********************************************/
    // DAQmx Start Code
    /*********************************************/
    DAQmxErrChk (DAQmxStartTask(taskHandle));

    printf      ("Generating voltage continuously. Press Enter to interrupt\n");
    getchar     ();

    Error:

    if ( DAQmxFailed (error) )
    DAQmxGetExtendedErrorInfo (errBuff,2048);

    if ( taskHandle != 0 )
    {
        /*********************************************/
        // DAQmx Stop Code
        /*********************************************/
        DAQmxStopTask   (taskHandle);
        DAQmxClearTask  (taskHandle);
    }

    if ( DAQmxFailed(error) )
    printf  ("DAQmx Error: %s\n",errBuff);

    printf  ("End of program, press Enter key to quit\n");
    getchar ();

    return 0;
}

int32 CVICALLBACK DoneCallback (TaskHandle taskHandle, int32 status, void *callbackData)
{
    int32   error           = 0;
    char    errBuff [2048]  = {'\0'};

    // Check to see if an error stopped the task.
    DAQmxErrChk (status);

    Error:
    if( DAQmxFailed (error) ) 
    {
        DAQmxGetExtendedErrorInfo   (errBuff,2048);
        DAQmxClearTask              (taskHandle);
        printf                      ("DAQmx Error: %s\n", errBuff);
    }

    return 0;
}

I would like to ask what generates the call to DoneCallback() function.  Thanks.