Usually, the Discount items will show up on List > Accounting > Items (Type: Discount). But Partner Discount won’t show up there. Because they are built-in records that have a negative internal ID which is hidden from the item list and Saved Searches.
Partner Discount is a Discount for Sale Item with Internal Id as -6
The following is the link that can be used to open the Partner discount in a NetSuite account.
https://system.na1.netsuite.com/app/common/item/item.nl?id=-6&whence=&e=T
Built-in items are intentionally hidden from the item list and saved searches to prevent users from editing it but they may still be accessed by changing the internal id in the URL. These Built-in Items can be used in transactions.