As a beneficiary of the StarHot rate (my friend's family member works for SPG so we travel together when possible), I can say from the half dozen experiences that you will need the form (a numbered certificate) that states the person checking in, the actual SPG employee, the property the employee works at, the name of their supervisor or manager of the property and their signature, the property the certificate is to be used at, the reservation number from the StarHot site, the date(s) of stay, along with a "stamp" that makes it official. I think there's other info on there, but you get the idea.
That document is collected upon check-in. Failure to produce it will result in a cancellation/no show of the reservation, along with the standard first night charge (at least that was what I have been informed).
This rate is for the benefit of the employees of SPG, not for the general audience. In addition, it usually provides a 50% discount off food and beverage, which makes a lot of the restaurants in the hotel cheaper than outside eateries. No points are awarded for such stays, and status benefits (i.e. upgrade) will not apply when using these rates, at least that is the standard procedure.
I've been able to stay in the Paris Westin for under $100 a night when the lowest rate available was over $450. There is also a limit on the number of days the rate is good for, i.e. you cannot be there for 2 weeks on that rate. It's a great benefit for SPG employees and they certainly deserve such a perk.