local skew and global skew

*  during CTS, when you choose global skew ,  skew = longest path - shortest path
    local skew, skew = endpoint - startpoint

*  local skew : source and destination flop insertion delay is called local skew.
    global skew: Max insertion delay minus Min insertion delay is called global skew.

*  the EDA speaks also of the useful/local/global skews
    useful is as you called the local skew
    the local skew is the skew in a sub-group
    global skew is the skew that include all clock pins.