Jagwire makes similar ferrules. If I remember correctly, they are for certain frames where the cable enters the frame tube.

 

Use the top one at the cable end on the derailleur, it helps reduce friction as there can be quite an angle from the cable housing to the actual securing bolt. Prevents the cable isn't rubbing on the bare metal of the derailleur.



This pic below doesn't have one.

 

Thanks. This is super helpful. Right after I posted this question, I found Shimano's Dealer Manual for the shifter (available here as pdf: https://si.shimano.com/en/pdfs/dm/MASL001/DM-MASL001-03-ENG.pdf), which has this semi-useful diagram and chart (screenshot below).

The diagram is still kind of ambiguous to me as to whether Shimano recommends the "sealed outer cap" or the "sealed outer cap with tongue" at the derailleur end, but your explanation makes the most sense to me and this helpful video by LoveMTB (at 11:20) also shows using the outer cap with tongue at the derailleur end, so that's what I'll do.

Thanks again!!!

 

Top one keeps the dust out. Sorry for the potato pic.

 

Guess you can base it off how much outer cable you have. If your bike runs an outer cable the full length then there's no where else it'll be useful. If you have the older type of guide that expose the inner cable on say the length of the top/down tubes or stays then it'll be useful to prevent dust ingestion.

 

I've never done this, and I don't have one of those derailleurs. So I also don't know how big the problem of the rubbing actually is. If it's a big problem, doesn't the plastic get filed away by the cable, too? and then potentially cause an additional obstacle?
I wonder why Shimano designed the cable routing the way they did. I guess there's a reason and it seems they accept the rubbing.

 

The idea with these is to keep out water and other contamination when running shifter cables with interrupted cable housing. The top one (or C in figure 3) goes on to the end of the housing and into the stop on the frame. D in figure 3 goes on the other side of the stop on the frame, in the gap where there is no cable housing. Just like bikedog's picture above. Where I have used these seems to work. I have less gummed up cables compared to 20+ years ago when I didn't run these.

Remember when we used to argue about which location was best for the cable routing. Top of top tube, bottom or top tube, or like yeti side of top tube, or maybe on the downtube.

 

Here they are in use on one of my bikes.

 

Thanks for everyone's input! I think everyone has great thoughts on this, and the ambiguity is all thanks to Shimano's poor documentation. Here's on additional piece of info that may help. I messaged LoveMTB, the Youtuber who made the video linked above, and he pointed me to another of his videos (linked below) that clarifies what he did.

He noticed that the XTR derailleur comes with an anti-friction tongue, which is just the tongue without the endcap, so he purchased that tongue and installed it in his XT derailleur. In my opinion the endcap with tongue can serve the same purpose, so unless I get more info to the contrary, I'm going to try using it there instead of separately ordering the anti-friction tongue.