The Shimano OT-RS900 Rear Derailleur Loop is a 240mm section of RS800 housing with an alloy derailleur ferrule and plastic inbound sealed ferrule. It is specified for use with Shimano's R9100, R8000, RX800, R7000 series Shadow style rear derailleurs. If you are running one of the 2nd generation 11-speed road rear derailleurs, you NEED this section of housing for proper function. The entry angle into the adjuster on the derailleur and smaller loop dictates the housing be more flexible than the OT-SP41 housing used elsewhere on the drivetrain to reduce drag and ensure proper function. This loop will work properly with the Polymer coated cables, as well as the Optislik cables (which are stock with R7000 drivetrains).

The final small part is called the "Cover with tongue (nose cap)". This small black part is a wear item used with most 2nd gen 11-speed Shimano mechanical rear derailleurs to reduce cable fray and is designed to be replaced with the polymer cable. It is the first piece that goes into the cable adjuster or stop on the rear derailleur. In the photos it is the smaller black insert with the barbs.

Helpful information: When you buy a Shimano R9100, R8000, RX800, R7000 series rear derailleur, this loop is now included. It appears Shimano has stopped including it with the cable set that comes with the Dual Control Levers to avoid confusion. Shimano also sells a complete Polymer cableset that includes this OT-RS900 loop, under their part number Y0BM98010 (Black), and a second Polymer cableset (Y63Z98910- Black) that does not include this loop.