Short answer:
Read your owners manual.
Long answer;
Kubota recommends either their premium SUDT2 transmission oil or their regular UDT product.
SUDT2 is a superior formulation especially well suited to HST transnissions and strongly recommended by Kubots.
UDT is a conventional universal tractor transmission fluid (THF) better suited to gear drive machines.
For all practical purposes SUDT2 is unique but UDT is indistinguishable from a host of other THFs.
Let the fun begin.
Dan