We are using grid with columnMenu: true setting then filtering menu is available on the second level of column menu.
In this example https://dojo.telerik.com/ofurEwaK/3 traditional filtering is applied for Product Name - user can specify up to 2 conditions.
In comparison https://dojo.telerik.com/ofurEwaK/4 allows for checkbox filtering - flag multi=true - user chooses by clicking checkboxes.
My question was about having both possibilities of filtering. This could be possible by adding new operator (IN_LIST) to the list of existing ones: eq, neq, startwith, ..., IN_LIST. After this operator has been selected user would be displayed a list of checkboxes representing possible values of product name. So it would be possible to display products with names starting with 'Queso%' and additionally products [ 'Icura','Tofu'].