Automatic ban duration.

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As Squidward mentioned, the ban is to get them to come on the forums and explain themselves.

Its more of an indefinite ban (rather than a true permanent ban).
Meaning it never expires but it can be set to the correct ban duration once they come on forums and appeal it.
Keep in mind depending on the type of auto ban (i.e. name spam vs bunny hop vs full spinbot) the RBD chosed by the admin will vary.