Hi @chuanlin,
This is explained in the footnotes:
Pulses with width <33ns are always filtered out by the internal filter.
Pulses with width >100ns always pass the filter.
Pulses with width in this range (33ns; 100ns) can be either filtered out or they can pass.
In other words, the LOW level condition on the reset_b pin must be > 100ns to ensure the MCU gets reset.
Regards,
Daniel