Hey Anjeanette,
Seth is correct.
I will say that the College of Science & Engineering has the most REUs that I know of. When I was in the department of Computer Science & Engineering, we had both an REU Site and 20+ REU supplements.
Historically, there was much debate about how REUs should be paid (for the supplements). It is imperative that you pay an REU separately from any other job record they may have. Most payroll appointments in HRMS are made in company UMN. All fellowships have appointments in UNS (as Seth was saying). Here is what the program announcement language is for REUs:
"The REU experience is a research training experience paid via a stipend, not employment (work) paid with a salary or wage. In this case, the student's training consists of closely mentored independent research. For administrative convenience, organizations may choose to issue payments to REU students using their normal payroll system. The funds received by students may be taxable income under the Internal Revenue Code of 1986 and may also be subject to state or local taxes. Please consult the Internal Revenue Service (IRS) for additional information. Students might find the IRS's "Tax Information for Education" website to be particularly helpful."
Payroll company UNS is used for stipends.
Please keep in mind if you are awarded the REU supplement, upon hiring your REU, it should be communicated to them that they are being paid a stipend, not a salary, and therefore no taxes are withheld and they are responsible for the tax implications of this stipend.
Additionally, I have also seen where this REU stipend has affected the student's financials. Meaning, from a student financial perspective, receiving an REU stipend counts as "income" towards their overall financial picture and can result in student loans needing to be refunded to the lender.
I am glad you raised the issue of REUs because it is not a well known issue around the U. I am happy to share what I know with anyone who has questions.