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3 of 4 clarify userids

On Android, the notion of a home directory is not apparent to the user, and it is designed for only one user.

The userid and groupid mechanisms are used to enhance application security. Each application has its own userid, and it is the job of the application to appropriately secure the data, which may be in the cloud or stored locally.

The model is intended to work even for applications which need to protect the data from the user. Think of a medical records application, which tries to prevent inappropriate use of the data.