A clipping mask and layer blend mode are two separate things. In certain pictures, and with specific layer values, the end result may look similar but they work very differently. The clipping mask, restricts the upper layer to the non transparent areas of the layer below it i.e. the layer it is clipped to. The blending mode changes the way the pixel values in the upper layer interact with the values in the layer(s) below it. In the case of screen, it lightens the combined result (mathematically it inverts the value in both layers, multiplies them, then inverts the result). You can, of course, combine a clipping mask and a blend mode.
Dave
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