The threshold is a fixed value that is set either at global level or at volume level. You can check the values by running:
dfm option list volFullThreshold
for the global level, and
dfm volume get <volume-name-or-id>
for the volume level.
You can change it using "dfm option set" or "dfm volume set" appropriately.
As Adai said, when the last sample value from SNMP exceeds the fixed value of the threshold, an event will be raised saying the volume is "full".