'Render', 'Rendering', 'Slot' in Blender
So, what are 'Render', 'Rendering' tabs for and how 'slots' appear. In 'Rendering' tab we can choose 'Render Engine' its parameters and set some 'Viewport' options and choose 'Slot' for next rendering output. From 'Render' we can launch rendering process itself so it will use setting configured in 'rendering tab' and that configs willl go to appropriate slot, that way you can keep mutiple configurations 'for render'. So in flow 'Render' goes after 'Rendering' even it goes before on panel.