The mask area is allowed to bind parameters, but why not？

Rhino.Inside Revit
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2

What are you trying to do? The masking region selected is a Detail Item Category.

The Masking Region Category is a system category and isn’t going to be on the list of bound parameter categories. You will need to add your parameter to the Detail Item Category.

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3

Got it,Thank u

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It’s still binding

5

What is the behavior you are looking for?