28 CFR 541.25: Notice received when placed in the SHU.

You will be notified of the reason(s) you are placed in the SHU as follows:

(a) Administrative detention status. When placed in administrative detention status, you will receive a copy of the administrative detention order, ordinarily within 24 hours, detailing the reason(s) for your placement. However, when placed in administrative detention status pending classification or while in holdover status, you will not receive an administrative detention order.

(b) Disciplinary segregation status. When you are to be placed in disciplinary segregation status as a sanction for violating Bureau regulations, you will be informed by the DHO at the end of your discipline hearing.