3000 for jets is issued when:
a) The jet will be given a turn instead of the SID;
b) It is departing from the "non-conforming runway" i.e. it will turn towards the other departure runway;
c) It is departing at the same time as another aircraft is doing b)
d) It might be less than 3 miles in trail of the previous jet departure which will provide another form of separation.
HTH
DC
British accent in the tower
