IsMirrored property - what is this?

I'm searching for a way to determine if control values have changed.
For some it's easy, such as TextBox.Modified. But looking at the
radiobutton I come across IsMirrored. Curious, I look to the help. Not
very helpful. So I look to the newsgroup. Not mentioned. So now I ask.
What does the IsMirrored property tell me?
.

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