The heater resistor limits current going to the heater filament, which is on pins 9 and 10. As the filament can also be modeled as a resistor, the polarity is unimportant, and DC or AC voltage may be used. As the name suggests, it heats up.
Like many tubes, the A68KJU96X is designed to have a heater voltage of 6.3V. This voltage is a function of the resistance of the heater filament as well as the current-limiting resistor.
The heater resistor for the MS9-29 is located at R532, and is tucked between the LOPT (Flyback) and the heatsink.
On the MS9-29N22, MS9-29T, and other submodels that are intended for use with the A68KJU96X tube, the heater resistor is a 3.3 ohm resistor.