Quoted from:http://www.saga-gis.org/en/index.html

The tool can be used to reclassify the values of a grid. It provides three different options:(a) reclassification of single values(b) reclassification of a range of values(c) reclassification of value ranges specified in a lookup tableIn addition to these methods, two special cases (NoData values and values not included in the reclassification setup) are supported.With reclassification mode (a) and (b), the 'NoData option' is evaluated before the 'Method' settings. In reclassification mode (c) the option is evaluated only if the NoData value is not included in the lookup table.The 'other values' option is always evaluated after checking the 'Method' settings.The tool allows one to define the NoData value of the output grid (header): by default, the output grid gets assigned the NoData value of the input grid. But it is also possible to assign a user defined NoData value.