Apart from row selection RadGrid supports the selection of individual cells and columns in the grid table. The cell selection functionality is controlled through the ClientSettings.Selecting.CellSelectionMode property:
SingleCell switches on the single cell selection.
MultiCell turns on multiple cell selection.
Column gives you the opportunity to select all the cells withing a given column by clicking on its header or using the column selection API (server and client side).
MultiColumn takes the column selection one step further and equips the grid with the ability to support multiple column selection.
N.B.: The selection state of RadGrid's cells is persisted on postback but not on rebinding of the grid.
To select cells in DetailTables of Hierarchical RadGrid you can follow the approach from Select cells in RadGrid with Hierarchy Code Library article