Switching frequency is the frequency at which the converter switches at. Outside of spread-spectrum variation, this is fixed. This is the frequency of the internal/external CLK depending on how SYNC is connected on the MAX20004
Frequency of LX pulses or output switching frequency is the frequency of the LX pulses that come. This can vary in Skip mode depending on the load and should approach the switching frequency as load current increases. In FPWM steady-state, the frequency of LX pulses should be identical to the switching frequency.