Data Transport between screens and ABAP programs is automatically handled by the system.

The First data transport occurs after all the PBO Modules have been executed and prior to a screen being displayed. The system will transport the contents of the ABAP work area in the program into the corresponding screen fields.

The Second data transport trigger is done once all the data entry is completed by the user and before the first PAI Module is executed. The system will copy the contents of the screen fields back to the corresponding ABAP work area in the program.

The Select Single Statement is used to ensure that only one row is read from the Database. The Search is terminated as soon as the that row is found with the required criteria. Select Single will definetly produce better performance then SELECT … UP TO 1 ROWS  … END SELECT when you have something is search for in the where clause.

Use Transaction ME01. Enter the Material Number and the Plant Data. Enter source list records, validity period, period of time material is procurable, Vendor Number, responsible Purchasing  Organization (or number of the Agreement or Contract), PPL (if the material can be procured from another plant), Fixed Source (?), MRP control.

Also, a check should be done to see whether any source list records overlap. To do so, choose Source List –> Check.

When you are posting a Goods Receipt for a PO, you can also enter items that you want to return to the vendor. To do this, you no longer have to reference the purchase order with which the goods were originally delivered. From the item overview, choose 161 (Return for PO) as the default value for the Movement Type. Enter the data for the return item(s) and post the document

The purpose of the source determination process is to automatically assign a source of supply to a purchase requisition. If the requester has set the Source Determination indicator in the requisition header, the system searches for possible source of supply for all the items of the requisition. If the system finds several valid sources for an item, the decision process can be supported by a price simulation or by reference to data from the vendor evaluation facility.
To have the system assign a single unique source during the automatic source determination process, a source list and/or a quota arrangement can be made use of.
In the source list, it is required to specify which source of supply is valid for a certain period of time. It is also possible to specify a preferred source as fixed for a certain period in the source list. In automatic source determination, the system will then suggest just this source, even if other potential sources exists.
With the aid of a quota arrangement, it is feasible to have material requirements arising over a certain period divided up among different sources on the basis of quotas. Quota arrangements determine which source of supply (maybe from among several possible ones) is automatically assigned to a purchase requisition.

Learn more about source list in this training document.