The session classes allows some keys to be reserved so the user of the module cannot access or change those particular keys. This allows a module writer to use the session store to store important variables which the user of the module should not have access to.
An example of an application which uses these features is the web.auth module which sets information such as when the application was started and when it was last accessed. It reserves these variables so that if a user is using the same session store for a different different purpose within the same application they cannot over-write these variables.
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