Shield Drones 9th - What special abilities do they have?

First up, I'm new to learning the rules of 40k and tau, so apologies if I've missed something simple:MV4 Shield Drones seemed to be popular, but nothing on the datasheet of the 40k app says what they do differently to the other drones and why they're more expensive (I understand there was a nerf to points). Marker drones have the same stats and abilities on the sheet but come with a Marker light. I've scoured the forums and people have mentioned 'feel no pain' but I just can't see where this rule is coming from?Can someone explain the difference and more importantly the source of the difference? via /r/Tau40K https://ift.tt/37zo47a