Picking notes is a useful feature that allows the pickers to make notes regarding product issues they find during the picking process.
This can for example be inventory notes like that inventory does not seem to match the recorded inventory in Shopify, or that it needs refilling.
This is set up in Settings→Picking notes. In order for this to work, the email(s) needs to be specified there. After this, a picking notes icon 
will appear at the end of the product line for order check and on-screen batch picking.
The note is sent by email to the specified recipient(s) when the batch pick is completed, or the order scan is completed, save and quit or marked as incomplete.
You can also edit this in Actions menu→Edit picking notes
The difference between packers note and picking notes is that packers note refers to an order, and picking notes refers to products without any reference to a specific order.
By clicking on the icon after the product name, a dialog will appear:
