When I need to implement ITransparentCollection

[JarJarD3]

I am creating a dialog utility (like TopLabelDecorator) and I am wondering why to implement ITransparentCollection.
And how to implement it properly?

I have used Fonts and Brushes without it a lot in plugins - but this is not a plugin but draws in the same places as plugin would do it.

TopLabelDecorator return Fonts and BackgroundTextures but not Brushes.
It seems to return both null and non-null values.