There have been a few issues with site-wide deals recently, so I hope this clarification helps.
Site-wide, or store-wide, deals are coupon codes or sales that affect the ENTIRE store (exclusions like sale items or premium products are ok). If a code is 20% off your purchase, it's likely sitewide. However, make sure to read the details on the deal to make sure it is valid on the majority of the store.
Valid sitewide deals:
20% off your purchase (Sale items exluded)
Get 10% off your purchase (Premium brands excluded)
Invalid sitewide deals:
Get $10 Off your Magnolia Home purchase at Target
Magnolia home is a brand at Target, so getting a deal on this brand is not a site-wide discount
Free shipping on Apple at Amazon
Apple is a brand on Amazon, so this discount will not work on the entirety of Amazon
I hope this clarification helps!