Type.missing

I'm learning office handling by C# but there are not full of references.
So I've just found by Google and MSDN how to access Office Programming by C#.
I wondered the meaning of Type.missing when I made a sample program like Excel Chart.
Finally,l I found it's meaning:
In the above code snippet, Type.Missing is used. This is because the above Excel method
requires multiple optional parameters that describe the behavior of the Excel chart you are
creating. In C#, you can only pass the Type.Missing value for parameters that accept reference types.
I think office programming by C#  is good.
that's all.