We use the built in fields of SO in PO lines and PO in SO lines to tie a SO and PO together.  In the lines of both purchase order and sales order you can enter the related purchase order or sales order.  Based on your comment are you wanting to print the SO picking sheet from PO? Are you trying to print multiple sales order picking sheets at once?  Are you trying to print SO picking sheets based on Purchase Orders?

The process of printing the picking sheets is what I am trying to figure out.  At first it seemed like Sage/our consultant says the "normal" process is to print picking sheets once or a few times throughout the day.  Our item code logic might not allow it, so yes we use the built in fields on SO and PO to tie the orders together.  However, I don't see a PO filter in the SO picking sheet list.  Which is brought me to the idea of the UDF to only print picking sheets for a certain PO.

You are correct that while on the sales order line you can enter the PO that is linked to it but this field is not included in the search criteria. You can create a UDF in the  Sales Order Detail and the Sales Order Picking Sheet Works table and make the source the PO Number field in Sales Order Detail. This will add it to the selection criteria.

Thinking about ways to help the picker know what needs to be picked (inventory vs out of stock items), is there a way to include the "available qty" on the crystal report for the picking sheet?  That way the picker knows in they have available inventory to pick?

Sure, you just need to add the fields into the works table for On Hand and On Sales Order and create a formula to calculate the available qty 

Adding a field to the work table does not affect the report's Selection grid.