problem about Trig. skew rise/fall skew

 In the "Synthesizing Clock Trees" chapter of the User Guide, there is a section called "How CTS Calculates Skew Values" which explains this. The triggered edge skew is probably the one you want to look at.

 It may depend on what you're looking at the skew for. As long as we're meeting timing, we don't worry too much about the skew itself. In fact, we often make use of "useful skew", where the clock is intentionally skewed more than the target in order to meet some timing paths. Of course you still need some sort of skew target to build a good tree to start with, but the skew number in and of itself may not be that important. Are you having timing issues that you think are related to the clock skew?