What is Shipment Order #?

What is Shipment Order #?

Seems like shipping is overly complicated here.  It's probably to accommodate situations different than ours, but here are a couple questions:

1) Can't the system default to Packing Slip #1 rather than me having to enter 1 for every single package we ship?

2) When I manually ship (all USPS is manual at the moment) what is Shipment Order # supposed to mean?  It's a required field, but I have no idea what it is asking for, where it's used, etc.  Is it the Sales Order Number?  Could it default to that?

Appreciate any help on proper shipping workflow.