Defining a DefaultSymbol provides a 'catch all' to draw the features for which no ClassBreakInfo's have been defined. If a DefaultSymbol is not defined, the Layer features will not be drawn on the Map when there is no grouping defined by the ClassBreakInfo's. For example: assume that a Layer has numerical observations that range from 1 to 90. If three ClassBreakInfo objects were defined that covered the grouped ranges of: (#1) 10 to 30, (#2) 30 to 60, and (#3) 60 to 90, unless a DefaultSymbol is defined there will be no visual representations visible on the map for the numerical range of 1 to 9.

To see a full example of how to set and use the DefaultSymbol Property in a ClassBreaksRenderer review the ClassBreaksRenderer Class documentation.