Single object selection puts object selection in single selection mode, disables the normal dialog conducted by object selection, and causes the selection to return the first object(s) selected by a subsequent option.
This can be quite handy in a macro. For example, consider the following macro:
*^C^CERASE single
This macro terminates the current command and activates the ERASE command with the single selection option. After you select this item, you either point to the single object to be erased or point to a blank area and specify a window. The object(s) selected in this way are erased, and the macro is repeated (due to the leading asterisk) so that you can erase something else. Single selection mode leads to more dynamic interaction with AutoCAD.