When working with lot and serial numbers and creating a delivery (WH/OUT), Odoo selects and reserves the needed amount of goods with the corresponding serial numbers. 

You can then manually change the serial numbers (to the existing ones). But for a large number of goods, it would take you a long time. Contrary to receiving the goods (WH/IN) you can not simply copy-paste the needed serial numbers from an excel list.

(see documentation: https://www.odoo.com/documentation/14.0/applications/inventory_and_mrp/inventory/management/lots_serial_numbers/serial_numbers.html) 

In short: How to manually choose which serial numbers to deliver to the customer (override the once picked by Odoo) for a large number of goods.