By default, the CustomFormat property returns nothing, as the AllowCustomFormat property is False by default. Once the AllowCustomFormat property is changed to True, it creates a new custom format object, that can be accessed later using the CustomFormat property. The new custom object copies the attributes of the DefaultInsideZoomFormat object. Use the AllowCustomFormat and CustomFormat properties to define units with different appearances, including labels, background or foreground colors, and so on. If the AllowCustomFormat property is false, the inside zoom units use the DefaultInsideZoomFormat property to define the look and feel of the inside units. If the AllowCustomFormat property, the CustomFormat property defines a custom look and feel for the inside unit. The CustomFormat property retrieves nothing, if the AllowCustomFormat property is False. 

The following screen shot shows the the units using the same format ( June 23 and June 24 ), AllowCustomFormat property is False:

The following screen shot shows the the units using the different formats ( June 23 and June 24 ), AllowCustomFormat property is True: