关于Liberate的使用问题

1237

在Liberate中关于setup和hold的介绍中有这么一段话The failure criteria can be a degradation in delay greater than constraint_delay_degrade (10%), a degradation in slewgreater than constraint_slew_degrade (50%), or a glitch on the output greater than constraint_glitch_peak (10% of Vdd). By default, flip-flops use delay degradation for both setup and hold, and latches use delay degradation for setup and glitch peak for hold. Flipflops can use glitch peak for hold by setting variable constraint_glitch_hold.
没有看明白是什么意思，怎样failurecriteria？和哪个标准做比较呢？
求助！！