Hit testing lets you find the character cluster that a co-ordinate is either directly over and/or closest to. This can be used to be build a range selection feature and/or as part of a more comprehensive editor.
// Hit test a mouse co-ordinate for example var htr = tb.HitTest(x, y);
The co-ordinates passed to the RichString.HitTest and TextBlock.HitTest methods must be relative to the top-left corner of the object. (ie: you'll probably need to adjust the co-ordinates by subtracting the top-left position of where you're displaying the text block).
The returned HitTestResult structure describes the line and code point cluster the point is either directly over and/or closest to.