Copy control fields

When we configure copy controls we have a field "COPY QUANTITY" this is left blank when copying from sales order to another sales order, from QT to OR we make +ve and from contract to Return we make it -ve. Can anybody explain me the logic behind this. How is this field useful?

Also in copy controls there is a field called "Copy product selection"can body explain me the functionality of this field.

Thanks in advance.

Venkat