If you want to set the start position to the first character in the control's text, set the start parameter to zero. You can use this method to select a substring of text, such as when searching through the text of the control and replacing information.
SelectionStart returns the lower value of the current selection. For example, Select(10, -4) selects the text from character position 6 through character position 10. If you now get the SelectionStart value, it will be 6 rather than 10.