Some issue on sequence diagram you should know
範例圖片：
圖一：

圖二：

圖二的sample code：

procedure dispatch
  foreach (lineitem)
    if (product.value > $10K)
      careful.dispatch
    else
      regular.dispatch
    end if
  end for
  if (needsConfirmation) messenger.confirm
end procedure

練習用一下英文描述東西，希望能夠表示出我想說的意思...
1.Each participant is an instance initated by class entity in class diagram

2.Lifeline can be started with newing an instance of object, can be delected by self or other objects.

3.Activation can define spending time of the operation of the participant . It also can define all processes in some operation.

4.By designing Operators(like loop, alt, opt,etc...), sequence diagram can let model and code more closely.