Under the Render Selection submenu you will see that there are many different conditions than you might want to render or not … you must select all the conditions that you want to be applied prior to invoking the render selection command
Heres an example: suppose I place a clip in the timeline that FCP can play back in full quality without need for any render whatsoever, and then I place next to it a clip that FCP can play back only in preview quality and finally I place one that can not be played back at all without rendering. and I then select all three and choose Render Selection … how should FCP handle this? should it render all clips even though one of them actually doesn’t need rendering at all, and another can be previewed anyway without render? FCP allows us to control what is and isn’t rendered by any given command through our choices in the Render sub-menu’s. So in this case I could choose that Render Selected only applies to Needs Render clips, or to Needs Render and Preview quality etc