One afternoon my colleagues asked me about something in his AutoCAD.
He was wondering why when he was using command copy and selecting an object, he cannot select multiple object, whenever he select another object, the previous object he was selected were removed from the selection set.
After looking to what his doing I found out that it was the case when the PICKADD value is set to 0 (turns off), instead of its default value which is 1 (turn on)
When the value PICKADD set to 0, the objects most recently selected become the selection set. The previously selected objects are removed from selection set. But by pressing SHIFT key on your keyboard while selecting you can add more objects to the selection set.
however the value of PICKADD set to 1, each object selected, either individually or by windowing, is added to the current selection set. To removed object from the selection set press SHIFT, while selecting.