I believe it refers to the Leopard Upgrade DVD that is provided with systems that shipped with Tiger installed after Leopard was released. The upgrade disc requires an existing version of OS X already be installed before you can use it to upgrade to Leopard.
It says "Tiger" on the box, so it doesn't look like it has anything to do with Leopard. Possibly it's the same idea applied to the jump from Panther to Tiger.
The picture of this eBay item is very clear -
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