I don't know how can I translate this feature in english, so please excuse me it's already asked.
I explain myself. Sometimes, inside a text, I can have 2 colors (black and white generally) and I want to overprint the black text only over the background (in french, I say that I want to avoid the "défonce", it's mean that I want to avoid the white text on the background below the black text). It's a problem only if I have 2 colors (or more) inside my text because if not I can use "Multiply" effect on the layer in order to obtain the same result. You can understand why it's a issue when I have a white text.
Am I clear?