When IsTabStop = False the focus is going through :

 *  RadAutoCompleteBox
 *  RadComboBox (when IsEditable is set to True)
 *  RadNumericUpDown
 *  RadMaskedTextInput
 *  RadDatePicker
 *  RadDateTimePicker

Available in the R3 2016 Release.

For RadAutoCompletebox, RadComboBox, RadDateTimePicker and RadNumericUpDown, the TabNavigationExtensions.IsTabStop attached property should be used to indicate whether the control is included in the tab navigation cycle.

For RadMaskedTextInput, MaskedInputExtensions.IsEditorTabStop attached property should be used.