How are the Packing List details related back to the SO delivery schedule?
Q.  How are the Pack List details related back to the Sales Order delivery schedule?

A.  Shipments are tied to the SO line item and NOT the SO line item schedules, so you cannot create a direct connect.  This is to allow users to ship against multiple delivery dates on a single packing list without creating multiple line items.    So you will NOT find a corresponding unique field in these tables.  User will need to use some type of FIFO to match shipments against SO line items and apply them in order to the SO line item schedules. 
 
For example:

SO                   SO Item          Due Date        Qty Due
0000000052        1                  02/05/09              10
0000000052        1                  02/15/09              10
0000000052        1                  03/01/09              10                   

SO                   SO Item          Ship Date       Qty Shipped Packing List
0000000052        1                  02/15/09              15                0000012345
0000000052        1                  03/01/09              15                0000012359

Would look like this:

SO                   SO Item          Due Date        Qty Due          PL Number    Shipped Date                    Ship Qty     On-Time         % On-Time
0000000052        1                  02/05/09              10                0000012345        02/15/09                               10                N                       0%
0000000052        1                  02/15/09              10                0000012345        02/15/09                               05                Y                       50%
0000000052        1                  02/15/09              10                0000012359        03/01/09                               05                N                          0%
0000000052        1                  03/01/09              10                0000012359        03/01/09                               10                Y                     100%