Through this option, the user can define an attribute that tags uncoverage methods, types or assemblies.
NDepend will then take care of this attribute and compute coverage ratio adequately.
For example, if all methods of a type are 100% covered by tests except a method that calls a blocking method such as MessageBox.Show(), this method can be tagged with the uncoverable attributes and as a result, the type.NDepend.CodeModel.ICodeContainer.PercentageCoverage ratio is artificially maintained to 100%.
The NDepend.API offers the attribute class NDepend.Attributes.UncoverableByTestAttribute for default uncoverable attribute, but you can define your own uncoverable attribute and set this property adequately.
The name of the uncoverable attribute must be formatted "{namespace}.{typeAttribute}". For example: "NDepend.Attributes.UncoverableByTestAttribute"

Reference

IProjectCoverage Interface
IProjectCoverage Members
UncoverableByTestAttribute Class