This property specifies the column used to expand and collapse the tree levels, if property "tree level field" is specified. The default is column 0. Each grid column is identified by a sequential index, starting with 0 for the first column, 1 for the second column, and so on. Note that if this property is specified, then property "tree level field" must also be specified.
Please see here for an example of how this property is used.
Possible Values: Any valid column number