Levels lower in the list are considered to have lower precedence than levels that are higher in the list. Thus if you use the threshold filter set to ERROR, then only levels upto (and including ERROR : 1 to 7) will be output. (or DEBUG < INFO < WARN < ERROR < FATAL)
log4net.Filter.DenyAllFilter Drops all logging events.
log4net.Filter.LevelMatchFilter An exact match to the event's level.
log4net.Filter.LevelRangeFilter Matches against a range of levels.
log4net.Filter.LoggerMatchFilter Matches against a the start of the logger name.
log4net.Filter.PropertyFilter Matches a substring from a specific property value.
log4net.Filter.StringMatchFilter Matches a substring from the event's message.