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Alumni Access Campus Resources:
Case 1: An institution might want to keep information about its alumni in a guest system (in addition to, or rather than an alumni database) as many alumni continue to use campus resources for some period of time after graduation.
Case 2: Many Alumni return to campus as a graduate student or employee, and being able to keep an active "guest identity" with their originally issued campus identifier(s) and PII allows them to be transitioned into the student or HR system if they return to the university.