These are the short belts that connect the x and y motors to the axes. Official Ultimaker Spare Parts for the following printers: Ultimaker 2+ Ultimaker 2 Extended+ Ultimaker 3 Ultimaker 3 Extended Ultimaker 2 Go Ultimaker 2 Ultimaker 2 Extended...
This is a set of fully assembled sliding blocks on Ultimaker S3 belts. This part is compatible only with the Ultimaker S3.
Installation instructions are available here.
When replacing this part, you may also want to replace your short belts as well.