Controls how mouse-based lightpen selections work.
When set to the default of false, clicking the left mouse button without a modifier will move the cursor or highlight an area for copy and paste. Clicking the left mouse button with Alt pressed will cause a lightpen select operation at the screen location where the mouse cursor is, and the 3270 cursor will not move.
When set to true, these are reversed. The left mouse button without a modifier will do a lightpen select; with Alt it will move the cursor or select for copy and paste.