Time (in seconds) that each engine has been idle.
In a typical environment, ASE does a good job of distributing processing to engines.
However, when there are more engines than tasks, you may see some engines with a large percentage of utilization, and other engines may be idle.
If there are not more engines than tasks, and the distribution between engines is still not fairly balanced, check to see if a strategy of distributing engine resources is in place (for example, utilizing execution classes, task-to-engine affinity, etc.).