Property CustomLists As IList

IList CustomLists {get; set;}

Return Value

The IRange.AutoFill and IRange.DataSeries methods support built-in lists consisting of long and short day and month names in the culture of the workbook set. These built-in custom lists may not be modified.

In addition, custom lists may be specified for the workbook set with this CustomLists property. This can be a single list such as an array of strings or it can be a list of lists to specify more than one custom list.

Reference