This is a spacer that centers the bearing in idig3d's tensiioners, and also centres the large guide washers.
The factory tensioner on the TronXY printers is below average, so as a new printer owner, one of the first upgrade parts you would print is a x and y belt tensioner. As with most of us, i decided to print the tensioners designed by "Idig3d".
The bracket in his design is 12mm wide, but the bearing with washers is only about 9.5mm wide. You could add some extra packing washers, but i also found the guide washers to be very loose.
This is a very simple fix.
Designed for use with a 0.4mm nozzle