I think this aspect must be standard, because it's on mine as well. Here's what the online 'help' says:
Being offered admission is a two-step process, and you are not considered to be offered admission until both steps are accomplished. Decisions and notifications come from two sources:
1. The graduate program: The graduate program will send a letter or an e-mail to inform you that you have been recommended to Rackham for admission. The graduate program will then communicate their decision to Rackham. Your status on the Web Application status page will remain 'Applicant' at this point. Contact the program for information regarding when decisions will be available.
2. The Rackham Graduate School: Rackham monitors requirements and procedures and certifies the admission recommendation as appropriate. If the recommendation is certified, Rackham will send a letter or an e-mail to let you know that your admission has been certified and you are being offered admission. Included in this letter is your University of Michigan ID number (U-M ID number). The Web Application Status page will show a statement of admission, and instructions for responding to the offer of admission. Once Rackham receives the recommendation from the program, you will be contacted within 10 business days.