DLL 을 불러 사용하는 함수인데요...

HMODULE hVctl= LoadLibrary("Ctl.dll");
if(hVctl==  NULL )
{
//Error handler code…
return ERR_LOADING_DLL;
}

LPVBIOCTL  g_pVbcontrol = (LPVBIOCTL)GetProcAddress(hVctl, "VbControl");
if(g_pVbcontrol == NULL)
{
//Error handler code…
    return ERR_PROC_IMPORT;
}
//The function of VBCTL_PARAM is self-defined structure

DWORD    dwVBVersion = 0;    //output parameter
VBCTL_PARAM    VBCtlParam;
ZeroMemory(&VBCtlParam, sizeof(VBCTL_PARAM));    //clear memory
VBCtlParam.dwMySize = sizeof(VBCTL_PARAM );    //size of the structure
VBCtlParam.dwCmd = VBCTL_GET_VBVERSION;        //control code
VBCtlParam.pOutBuf = (PBYTE)&dwVBVersion;    //the output Buffer
VBCtlParam.dwOutBufSize = sizeof(DWORD);        //the size of output buffer

이런 경우에... VBCTL_PARAM 이 어떻게 되는것인지 모르겠습니다.
The function of VBCTL_PARAM is self-defined structure 이게 무슨 의미이지요???