请解读这个宏

下面是BEGIN_SERVICE_TABLE的定义，那位高手能帮忙逐句解读一下，
Begin_Service_Table MACRO Device_Name, Def_Segment

IFDEF Device_Name&_Name_Based
 IFNDEF @@NextInternalID
    @@NextInternalID = 0
 ENDIF
 @@NextInternalID = (@@NextInternalID + 1)
 Device_Name&_Internal_ID   = @@NextInternalID + BASEID_FOR_NAMEBASEDVXD
 DefineVxDName Device_Name, %Device_Name&_Internal_ID
ENDIF

IFB <Def_Segment>
    BST2 Device_Name, VxD
ELSE
    BST2 Device_Name, Def_Segment
ENDIF
    ENDM
还有@@、%、&是什么，我好象没学过呀？