The S key is a convenient way to pop up a set of commonly used commands in the context of what you have selected. For example, select a part, press the S key and you get icons for Review, Assemble, Blend, Chamfer, etc. There is a different S key menu for each context. The best part is it's completely customizable. Go to Customize Toolbar, Menu, Keyboard, select the "Popup Floaty" tab and you can change what commands appear for the varius contexts.