LOCAL SKEW : The difference between the clock reaching at the launching flop vs the clock reaching the destination flip-flop of a timing-path.

Global skew : The difference between the earliest reaching flip-flop and latest reaching flip-flop for a same clock-domain.

Useful skew: Useful skew is a concept of delaying the capturing flip-flop clock path, this approach helps in meeting SETUP requirement with in the LAUNCH and capture timing path. But the hold-requirement has to be met for the DESIGN.

Local skew : The difference between the clock reaching at the launching flop vs the clock reaching the destination flip-flop of a timing-path.

Global skew : The difference between the earliest reaching flip-flop and latest reaching flip-flop for a same clock-domain.

Useful skew: Useful skew is a concept of delaying the capturing flip-flop clock path, this approach helps in meeting setup requirement with in the launch and capture timing path. But the hold-requirement has to be met for the design.