This indicates at which levels the Intermediate System is running and follows the replaceOnlyWhileDisabled behavior. It is a core architectural decision -- intra-area routing only (level1), backbone-only (level2), or a full border router (level1L2) -- that determines whether this box can carry inter-area traffic at all, but because of replaceOnlyWhileDisabled the admin must first disable IS-IS before changing it, so it cannot be flipped live. For example, converting a pure level1IS access router to level1L2IS so it can act as an area border router requires shutting down its IS-IS process first, changing this value, then re-enabling it.