DEVMODE private data in DDK unidrv sample

[remedius]

Hello!
I just want to understand DDK unidrv example. As far as I've read unidrv can
include two plug-ins. So,I try OEMDEV structure to send some data from one to
another. But when I add some more fields in OEMDEV stucture (I mean besides
OEM_DMEXTRAHEADER and dwDriverData, which are happened in DDK example), the
data stopped translated between the plug-ins. Is OEMDEV structure of a fix
size, or my private data should be described anywhere more?
To fill private members I use the following:

LONG APIENTRY OEMDocUICallBack(PCPSUICBPARAM pCallbackParam)  
{  
......  
PCBUSERDATA   pUserData = (PCBUSERDATA) pCallbackParam->UserData;  
POEMDEV   pOEMDev = (POEMDEV)pUserData->pOEMUIParam->pOEMDM;  
........  
pOEMDev->dwDriverData1 = 1;  
pOEMDev->dwDriverData2 = 2;  
pOEMDev->dwDriverData3 = 3;  
.......  
}