求解答operating condition相关的library cell 的rail voltage等概念

weip218

阅读ICC uer guide时遇到两处不懂的地方，已在下文贴出上下文并标红关键字。
第一处：rail voltage是什么概念？同一个library cell会有不同的rail voltages？power connection rails又是个什么概念？
第二处：同样的，一个library cell会有好几个rails？每一个rail对应的PVT都有可能不一样？奇怪啊，一个cell的某个library不就是根据不同的PVT所得到的，也就是说一个library cell应该是某一个PVT的library cell，怎么这里的说法好像一个librarycell 还可以有好几个PVT???

Resolving References With Operating Condition Mismatches
Before doing timing analysis, you can check whether your design contains any cells for which the link libraries do not have cells characterized with the correct operating conditions by running the check_mv_design -opcond_mismatches command.
By default, when the tool cannot find an exact match for the reference cell name and operating conditions, it tries to match the reference cell with the same name and the closest operating conditions using the following process:
1. Find the set of library cells with the same name as the reference cell.
Note:
If the cell instance is connected to a power net, reference cells whose rail voltages do not match the explicit power connection rails are not included in this set.
2. Identify the library cells in this set that match each of the following criteria, in order.
a. The PVT values of the library cell match the PVT values of the design.
b. The process, temperature, and voltage values from one of the rails of the library cell match the PVT values of the design.
c. The temperature and voltage values of the library cell match the temperature and voltage values of the design.


求大牛解答，谢谢：）

icfbicfb

只有pmk cells（level shifter)有2个rail， VDD、VDDI
header/footer 有2个rail： VDDG，VDD
其他 cell一般就vdd，vss，没啥好变的