Post Goods issue is done when physically the goods are moved from warehouse or plant after Delivery is fully picked.

When you post goods issue for an outbound delivery, the following functions are carried out on the basis of the goods issue document:·

1. Warehouse stock of the material is reduced by the delivery quantity· 2. Value changes are posted to the balance sheet account in inventory accounting·
3. Requirements are reduced by the delivery quantity ·
4. Goods issue posting is automatically recorded in the document flow

After goods issue is posted for an outbound delivery, the scope for changing the delivery document becomes very limited. This prevents there being any discrepancies between the goods issue document and the outbound delivery.

https://scn.sap.com/thread/912972

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Note added at 4 hrs (2015-10-21 23:02:51 GMT)
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Shipping 3: Post Goods Issue and Bill of Ladings

Once the products are loaded onto the trucks and ready to leave the plant, Warehouse Personnel will need to perform a Post Goods Issue (PGI). This tells the system that inventory has left the warehouse and SAP will therefore reduce the stock levels in the system.

This process will also automatically print the Bill of Lading document (see a Sample Bill of Lading)

http://www.sapinfoasap.com/post-goods-issue-and-bill-of-ladi...

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Note added at 4 hrs (2015-10-21 23:07:24 GMT)
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PGI (post goods issue) is nothing but confirmetion (sic!) of delivered quantity. PGI-Post goods issue is the last step of delivery/shipment processing, its the step where the ownership of the goods transfer to the customer and the stock is updated. - http://www.magnatraining.com/what-is-pgi-or-what-is-the-sign...