When creating sequence diagrams, using the Use Case Specifications, it is always troublesome to determine whether all sequences are to be drawn.
Also there are always parts in a use case that are shared amongst different sequences. To avoid creating an explosion of sequence diagrams, the UML 2.0 specification has created fragments.
I found an interesting (and short!) article on how to relatie alternative flows and sub flows into these fragments.
Check it out at: Managing-Complex-Interactions-with-Sequence-Fragments