Alex, a "softshell" describes an active clothing layer that is any combination of being hooded, waterproof, windproof, breathable and insulated, pick any 3!
An ideal softshell is something that replaces 2 or 3 active layers, e.g. like a baselayer/fleece and Houdini, and worn continuously thru a broad range of conditions saving pack space and hopefully some weight too. They work best at cool/cold/below freezing temps. When you think of a softshell as a replacement layer(s), then it gets easier to imagine how you would layer over it.
Many here have adopted the windshirt in combination with a wicking baselayer and/or light fleece as a flexible DIY "softshell" for 3-season and summer alpine use. But if temps are consistently on the cooler/cold side, you can replace these layers with a single softshell like a Rab Vapour Rise Lite to something as warm as a Patagonia Northwall in Winter.
BTW, I cannot imagine any scenario save for an emergency bivouac where it is a good idea to wear a down layer under a light GTX or eVent membrane-type rain shell.