FallbackValue vs. TargetNullValue

FallbackValue: Gets or sets the value when the binding is unable to return a value.
TargetNullValue: Gets or sets the value when the value of the source is null.

For instance, we have a label:

< Label Name="Label" Content="Value1" />

And a TextBox:
< TextBlock Text="{Binding ElementName=Label,Path=Content,FallbackValue=Value2,TargetNullValue=Value3}" />

We can test by doing the following operations respectively:

1.Do nothing,
2.Remove the Content="Value1"
3.Change Path=Content to Path=Content2

the text show in the Textbox will be:
Value1
Value3
Value2