Question

columns.Bound(model => model.Item).Filterable(false).Title("")
           .ClientGroupHeaderTemplate("<span> Item: <#= Item #> </span>");

How can I get the column values from item to be displayed in unique. That is, I don’t need the “Item” values to be display each time. Just once if it displayed, it should not display again since the task is mainly for grouping Detail Items in to “Item” header.
“Items” which are coming under “Item” header should have only 1 “Item” header and not for each row (each “Items”)

Answer 1

Did you try adding Hidden(true) ?

columns.Bound(model => model.CategoryItem).Hidden(true).Filterable(false).Title("") 
       .ClientGroupHeaderTemplate("<span> CategoryItem: <#= CategoryItem #> </span>");