What tool is used to set the chain gap for an XPLR rear derailleur?

There is a tool specific to the adjustment of the XPLR rear derailleurs, but you can also use a ruler. The chain gap adjustment for XPLR rear derailleurs is completed with the derailleur shifted inboard to the 2nd largest cog. Install the gauge onto the upper pulley, with the top half of the gauge behind the largest cog. Rotate the b-adjust screw until the appropriate gap is achieved for the cassette you are using - this distance is 11mm for 36T cassettes, and 7-8mm for 44T cassettes.