Where do the contents of the combobox on the Telerik demo ASP.NET Core Grid Custom Editing are coming from?

        [UIHint("ClientCategory")]
        public CategoryViewModel Category
        {
            get;
            set;
        }

.Columns(c=>{
             c.Bound(x=>x.Category).EditorTemplateName("ClientCategory"));    
})

@model Kendo.Mvc.Examples.Models.CategoryViewModel

        @(Html.Kendo().ComboBoxFor(m => m)
              .DataTextField("CategoryName")
              .DataValueField("CategoryID")
              .HtmlAttributes(new { style = "width:100%"; data_bind="value: Category" })
              .DataSource(source => source
                    .Ajax()
                    .Read(read =>
                        {
                            read.Action("ActionName","ControllerName");
                        })
                    )
              )
        )