By default, the AllowHeaderExt property is exCornerTopLeftExtHeaderTop Or exCornerTopRightExtHeaderTop Or exCornerBottomLeftExtHeaderBottom Or exCornerBottomRightExtHeaderBottom, which indicates that the top/bottom headers take the corners like in the following picture. The AllowHeaderExt property specifies the way the left/right or/and top/bottom header extends ( for specified pool). The Background(exPoolHorizontalHeaderBackColor)  property specifies the background color / visual appearance of the top/bottom header of the pool. The HeaderColor(exHeaderHorizontal) property specifies the background color/visual appearance of the horizontal header for the giving pool. The Background(exPoolVerticalHeaderBackColor)  property specifies the background color / visual appearance of the left/right header of the pool. The HeaderColor(exHeaderVertical) property specifies the background color/visual appearance of the vertical header for the giving pool. 

The following screen shot shows how the header is displayed if the AllowHeaderExt property is exCornerEmpty:

The following screen shot shows how the header is displayed if the AllowHeaderExt property is exCornerTopLeftExtHeaderTop Or exCornerTopRightExtHeaderTop Or exCornerBottomLeftExtHeaderBottom Or exCornerBottomRightExtHeaderBottom ( default ):

The following screen shot shows how the header is displayed if the AllowHeaderExt property is exCornerBottomLeftExtHeaderLeft Or exCornerBottomRightExtHeaderRight Or exCornerTopRightExtHeaderRight Or exCornerTopLeftExtHeaderLeft:

The following screen shot shows how the header is displayed if the AllowHeaderExt property is exCornerBottomLeftExtHeaderLeft Or exCornerBottomRightExtHeaderBottom Or exCornerTopRightExtHeaderRight Or exCornerTopLeftExtHeaderTop: