isolation cell

xyz1123456

-isolation_target {from|to)
guide上解释是：
from indicates that the rule applies when the power domain of the drivers of the specified pins is switched off.
to indicates that the rule applies when the power domain of the loads of the specified pins is switched off.

假设A是power off domain B是power on domain 。iso cell 在B中，A就是switched off。这样的话，cell的位置定了，方向肯定是A OFF 然后到B，那这个参数有什么用么？？