the one marked return will probably go to a dis box in the loft that runs all the other tv points.

So you go through your sky box etc, then you split the signal between your main tv and the return. this then gives you sky on all tv points. Some people will send the signal to the loft then back down to the main tv and other points so its not splitting it before the signal booster.

edit, just read the rest of your post and think you already have it sussed