What's the difference of TLD5099EP dimming between using PWMO and opening PWMO?
For an LED driver the difference is on the waveform of the output current.
Using PWMO connected to an NMOS the LEDs can be disconnected from the output capacitor. Then the system is able to produce a very sharp current waveform at the output. If the PWMO is left open the current waveform is prolonged even after the PWM signal goes to low level due to the discharging of the output capacitor.
The reason is that without the NMOS the output current is not interrupted according to the PWM modulation and it flows into the load until the output voltage of the DC-DC is lower than the forward voltage of the LED string.
This means that with the switch the system is able to replicate at output also very low duty cycle PWM modulation that are not possible without (the system has to recharge the output voltage to have the desired output current).