The HUP is called the Home Use Program. It's available to organizations who purchase Enterprise Agreements from Microsoft. Once they are eligible for the program, they are allowed to provide 1 copy to an employee at a deeply discounted rate. Usually $9.95 or $19.95. The reason for the program is to provide a way for employees to be able to work on files/documents from the office on their home use computer.
The Home Use Program has a few limitations. Namely...employees are only able to purchase 1 copy for their OWN personal use and "technically" if they discontinue employment with the organization they are expected to discontinue using the software (although enforcing that is very hard indeed).
Because of the high cost of software....both the OS and the office suite many people do obtain it through shady means.
What I find "very" unfortunate is that Microsoft encourages this poor behavior through so many of these programs which offer ridiculously low prices to "some" people but then such high prices to others.
For example, I understand a child discount or a senior citizen discount at a movie theater....but that discount is like $5 per ticket instead of $7.50. If Microsoft ran the program it would cost $42.00 for 1 adult ticket and $0.05 for a child. Or, the movie patron might participate in the "CriticNet" program designed for Professional Movie Critics...which for $29.95 would allow the Movie Critic to see whatever movie they wanted...multiple times....for 1 year and then allow them to renew for a second year for $9.95. But the expectation would be that if 2 people aged 25 came in to watch a movie on date night, it would be $42.00 x 2 or $84.00...for the 1 movie, 1 time.