Shift key is pressed, all cursor movement results in a current selection being modified. The range between the original cursor position and resulting cursor position is added to, subtracted from or laid upon the current selection, depending on its original configuration.
If multiple cursors are present in the window, all of them start new multiple selection range.
When “Single Selection mode” option is on, you can only make a single selection with keyboard. See also Selecting with Mouse section for information how to make a multiple selection with enabled “Auto drop selection” option.
Keyboard selection actions may be emulated by scripts using the methods in ICursor interface.