There are 2 modes of soft start that are usable in this chip: Current mode and voltage mode. Connection to the SLOPE pin determines what mode is used, and determines the value of the slope.
Connecting SLOPE to GND - Current mode SS, slope = ~60mV/µs
Connecting SLOPE to VCC - Voltage mode SS, slope = ~60mV/µs
Leaving SLOPE open - Same as VCC, Voltage mode SS, slope = ~60mV/µs
Connecting SLOPE to Resistor to GND - voltage mode when Slope pin is connected with a resistor in the allowable 30-150k Ohm range, to GND.