If you use ApplyICEOp to apply a compound, then you can open the compound PPG by clicking the ICE tree operator icon in the explorer. For example, in this screenshot, clicking the Bend operator icon opens the Bend PPG (Bend is an ICE compound).

If you create the ICE tree yourself and hook up the compound, you don’t get this behavior. That’s because ApplyICEOp knows what compound is being applied, so it can nest the compound node directly under the ICE operator node, and the operator PPG can inspect the compound parameters.