The first field on the block configuration page, Custom Visibility Settings, deals with the question of whether authenticated users should be able to customize which blocks are visible to them when they visit the site. The first option, "Users cannot control whether or not they see this block," means essentially that the administrator-defined visibility settings are to be honored, and the user will not be given the choice to enable/disable this block. The other two options, "Show this block by default, but let individual users hide it" and "Hide this block by default but let individual users show it," both allow users to enable/disable the block for themselves, but differ in whether the block is initially shown or not shown. (Users can enable or disable custom blocks from their user account editing page, user/uid/edit.)
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