The Hall interpolation start/stop frequency parameters corresponds to the Hall sensor frequency at which the calculated rotor angle transitions to/from a series of six discrete steps to/from a smooth triangle wave based on interpolating the time between Hall sensor edges.
The parameters can be used to reduce the chatter and noise associated will Hall sensor commutated motors when operating at low speeds. Typically, the interpolation start frequency is set at twice the value of the stop frequency to prevent rough motor operation. It is recommended that the lower the interpolation frequencies can be set the better. This is particularly true with high pole count direct drive motors.
This tuning process will require some trial and error in order to get the bike to feel right. The most important factors in determining how much chatter can be felt at low speeds are spoke tightness and how much power the controller is providing to the wheel. Indeed, for some configurations, it is not possible to completely eliminate low speed chatter.