The Long Trail Side-to-Side Challenge involves completing all 88 side trails of the Long Trail system. The official trail list is maintained by the Green Mountain Club: https://www.greenmountainclub.org/the-long-trail/side-to-side/. Finishing all of the trails in one go is an interesting logistical challenge that provides a unique cross-section of the mountains of Vermont.
There is no single defined route for the Side-to-Side. The only objective is to complete all the trails in whatever order is deemed most efficient. While the official distance is 166.1 miles, any FKT attempt will be well over 200 miles due to necessary out-and-backs and connecting sections of the Long Trail. The GPS tracks submitted are from the run I (Nigel Bates) completed in 6d 10h 48m 15s starting on 7/22/20, totaling 226.66 miles with 56,947' of elevation gain. From what I could gather, this is the first successful attempt to complete the Side-to-Side in a single push. The only other attempt for which I could find documentation was made by Guthook in 2011 (https://www.guthookhikes.com/2011/06/long-trail-side-to-side-part-1-lye-brook-wilderness.html), and he did not finish, nor is it clear he intended to.