As noted, Mc6 can be clicked because they have material names in them - they automatically pick what material - all you need is a current figure.

Mt5 must be dragged if you are not in the material room. Dragging over some polygon then identifies what material zone you want to load the shader into.

But - while in the material room, dragging is optional - simple double click or clicking load button is fine there, because when you are in the material room, there is a "current material". While in the mat room, dragging into the preview window allows loading into something other than the current material.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)