Hello @HelenMC and thank you for reaching out via our customer community. Welcome. You ask a couple of good questions that I am going to answer with best practice suggestions but strongly suggested.
When you need to populate the Discontinued Date with a current (not future) date for a currently published item, the best practice is to FIRST DELETE THE PUBLICATIONS FOR THAT ITEM. Sorry for the caps but it is just so important. Once deleted, you can then populate the Discontinued Date to de-register the item. If you were to populate the Discontinued Date with a current date before deleting the pubs, you will run into issues for your reporting and that of your trading partner.
There are 4 attributes that make an item unique and those same 4 attributes are what is used to register your item with the Global Registry. Those are GLN, GTIN, Target Market, and GPC. That means, even if other companies are publishing the same GTIN from their own GLNs, yours is unique because it came from your GLN. You could also let any of your contacts for impacted trading partners know you are discontinuing your items. Technically, when you delete a pub, it lets your trading partner know you are breaking synchronization.
