Tooltips generally apply to controls as a whole, not to parts of a control. So if you want a tooltip on a particular part of a control, the typical way to do it is to trap the MouseMove event of the control and determine where the mouse is and set the ToolTip accordingly.

In this case, you want to find a RowSelection, so in the MouseMove event, you would usegrid.DisplayLayout.UIElement.ElementFromPoint to determine the UIElement under the mouse at that point. If it's a RowSelectorUIElement, then you can set the tooltip you want. If not, you can clear the tooltip.