I think it is the y value of the texture?
void T_Rover:

efineMFDButtons()
{
SURFHANDLE mfd_buttons_surf;
mfd_buttons_surf = oapiGetTextureHandle(VCMesh, 4);
oapiVCRegisterArea(CDR1_PWR, PANEL_REDRAW_NEVER, PANEL_MOUSE_LBDOWN | PANEL_MOUSE_ONREPLAY);
oapiVCRegisterArea(CDR1_SEL, PANEL_REDRAW_NEVER, PANEL_MOUSE_LBDOWN | PANEL_MOUSE_ONREPLAY);
oapiVCRegisterArea(CDR1_MNU, PANEL_REDRAW_NEVER, PANEL_MOUSE_LBDOWN | PANEL_MOUSE_ONREPLAY);
oapiVCSetAreaClickmode_Quadrilateral(CDR1_PWR, CDR1_PWR_AREA.pt1, CDR1_PWR_AREA.pt2, CDR1_PWR_AREA.pt3, CDR1_PWR_AREA.pt4);
oapiVCSetAreaClickmode_Quadrilateral(CDR1_SEL, CDR1_SEL_AREA.pt1, CDR1_SEL_AREA.pt2, CDR1_SEL_AREA.pt3, CDR1_SEL_AREA.pt4);
oapiVCSetAreaClickmode_Quadrilateral(CDR1_MNU, CDR1_MNU_AREA.pt1, CDR1_MNU_AREA.pt2, CDR1_MNU_AREA.pt3, CDR1_MNU_AREA.pt4);
oapiVCRegisterArea(PLT1_PWR, PANEL_REDRAW_NEVER, PANEL_MOUSE_LBDOWN | PANEL_MOUSE_ONREPLAY);
oapiVCRegisterArea(PLT1_SEL, PANEL_REDRAW_NEVER, PANEL_MOUSE_LBDOWN | PANEL_MOUSE_ONREPLAY);
oapiVCRegisterArea(PLT1_MNU, PANEL_REDRAW_NEVER, PANEL_MOUSE_LBDOWN | PANEL_MOUSE_ONREPLAY);
oapiVCSetAreaClickmode_Quadrilateral(PLT1_PWR, PLT1_PWR_AREA.pt1, PLT1_PWR_AREA.pt2, PLT1_PWR_AREA.pt3, PLT1_PWR_AREA.pt4);
oapiVCSetAreaClickmode_Quadrilateral(PLT1_SEL, PLT1_SEL_AREA.pt1, PLT1_SEL_AREA.pt2, PLT1_SEL_AREA.pt3, PLT1_SEL_AREA.pt4);
oapiVCSetAreaClickmode_Quadrilateral(PLT1_MNU, PLT1_MNU_AREA.pt1, PLT1_MNU_AREA.pt2, PLT1_MNU_AREA.pt3, PLT1_MNU_AREA.pt4);
oapiVCRegisterArea(CDR1_LBUTTONS, PANEL_REDRAW_NEVER, PANEL_MOUSE_LBDOWN | PANEL_MOUSE_LBUP | PANEL_MOUSE_LBPRESSED | PANEL_MOUSE_ONREPLAY);
oapiVCRegisterArea(CDR1_RBUTTONS, PANEL_REDRAW_NEVER, PANEL_MOUSE_LBDOWN | PANEL_MOUSE_LBUP | PANEL_MOUSE_LBPRESSED | PANEL_MOUSE_ONREPLAY);
oapiVCSetAreaClickmode_Quadrilateral(CDR1_LBUTTONS, CDR1_LBUTTONS_AREA.pt1, CDR1_LBUTTONS_AREA.pt2, CDR1_LBUTTONS_AREA.pt3, CDR1_LBUTTONS_AREA.pt4);
oapiVCSetAreaClickmode_Quadrilateral(CDR1_RBUTTONS, CDR1_RBUTTONS_AREA.pt1, CDR1_RBUTTONS_AREA.pt2, CDR1_RBUTTONS_AREA.pt3, CDR1_RBUTTONS_AREA.pt4);
oapiVCRegisterArea(PLT1_LBUTTONS, PANEL_REDRAW_NEVER, PANEL_MOUSE_LBDOWN | PANEL_MOUSE_LBUP | PANEL_MOUSE_LBPRESSED | PANEL_MOUSE_ONREPLAY);
oapiVCRegisterArea(PLT1_RBUTTONS, PANEL_REDRAW_NEVER, PANEL_MOUSE_LBDOWN | PANEL_MOUSE_LBUP | PANEL_MOUSE_LBPRESSED | PANEL_MOUSE_ONREPLAY);
oapiVCSetAreaClickmode_Quadrilateral(PLT1_LBUTTONS, PLT1_LBUTTONS_AREA.pt1, PLT1_LBUTTONS_AREA.pt2, PLT1_LBUTTONS_AREA.pt3, PLT1_LBUTTONS_AREA.pt4);
oapiVCSetAreaClickmode_Quadrilateral(PLT1_RBUTTONS, PLT1_RBUTTONS_AREA.pt1, PLT1_RBUTTONS_AREA.pt2, PLT1_RBUTTONS_AREA.pt3, PLT1_RBUTTONS_AREA.pt4);
oapiVCRegisterArea(ALL_MFD_BUTTONS, _R(0, 0, 1536, 384), PANEL_REDRAW_USER, PANEL_MOUSE_IGNORE, PANEL_MAP_BACKGROUND, mfd_buttons_surf);