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The term useful skew just means that the clock latency difference between the sending and receiving flop (or skew) is actually helping (instead of hurting) the path to meet timing. The clock at the receiving flop arriving later helps setup time, if it arrives earlier it helps hold time.
Some tools claim to use useful skew during CTS to help close timing. This means that they are not balancing both launch and capture flop latencies to the same number, and the difference in latencies is designed to help the critical timing paths.