Alice appreciates the convenience of a PC-based electronic calendar, but to maintain her privacy, she has set strict permissions for her calendar file: She can add new appointments herself, but other members of her workgroup have read-only access. Others outside of her workgroup are not even allowed to look.

This configuration is fine at first, but one day Bob from another department agrees to collaborate with Alice. To allow this to happen, she has to give him access to her calendar data.

In this scenario, it is clear that the legacy Linux permission system has outlived its usefulness. To allow Bob to read the file with her calendar data, Alice can ask the administrator to move the new colleague into her own group, but this would allow Bob to view all the other documents produced by Alice's team. Another approach would be to temporarily set up a completely new user group with both Alice's and Bob's accounts as members. In this simple scenario, a temporary group might be an acceptable solution, but in a real-world enterprise environment, group management becomes far more complicated, and the habit of creating temporary groups on the fly can lead to too many groups with no good way of tracking them.

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