It's actually not "plus" It's T for Temp. Listed the description of both from the HB website below. If you want temp, buy T, if you don't buy the one without T. Reference Humminbird :: Fishfinder :: Depth Finder :: GPS Chartplotters :: Side Imaging Sonar Technology :: Marine Fish Finders

XNT 9 QB 90
Quadra Beam, 20/60/90 Degree, 200/83/455 kHz.

XNT 9 QB 90 T
Quadra Beam, 20/60/90 Degree, 200/83/455 kHz with built in temperature.