BLOCK_CACHE_PER_USER

The block can change depending on the user viewing the page. This setting can be resource-consuming for sites with large number of users, and thus should only be used when BLOCK_CACHE_PER_ROLE is not sufficient.

drupal/modules/block/block.module, line 52

<?php
define('BLOCK_CACHE_PER_USER', 0x0002)
?>