HUP is a benefit that Microsoft offers to volume licensees with Software Assurance. It allows business users to buy and use the latest version of Office to use on their home PCs/devices through a "low-cost" download. (Neowin says the Professional Plus 2013 SKU will cost about $9.95 per user via HUP.)
In other Office-related news, those who've been testing the preview version of Office 2013/Office 365 Home Premium shouldn't worry that their test versions will expire before Microsoft launches the final version of the product.
The notification dates were built into the preview version to be in-line with how subscription users will be notified that an Office 365 subscription or trial will expire and remind them to renew or purchase once the next version of Office is available in their market.
"The messages trial customers see now are essentially of the Customer Preview experience as the team tests the system," the Microsoft spokesperson said. "Trials continue to live on without people having to do anything."