Configuration for merge picking orders, here you can choose merge type and if you tick the 'Notify in Chatter' option then it gives notification of merged orders into the chatter.

'Merge Picking Orders' menu and tree view.

You can merge only draft/ waiting/ ready state orders.

To merge picking orders you have to select more then one operation.

You can merge picking orders which have the same operation type.

Merge picking orders wizard with the default selected merge type.

Merge picking orders wizard with the existing order type.

We merge the picking orders with the default selected merge type.

Here it creates a new order and cancels selected order

The merged picking order form view.
Merged orders notification comes into the chatter.

Enable 'Subtract Merged Quantity'.

Form view of the picking order.

So it opens merge picking order wizard with all records quantity & On-hand qty. You can subtract qty from this wizard.

Created order list view.

New generated picking order looks like below.

Old picking with remaining quantity.

Old picking with remaining quantity.

If you try to add qty more than order line qty then alert occurs.