“push” and “pop” to/from G4StackingManager accessed only by G4EventManager

Three “Last-In-First-Out” stacks (in default)

• “Urgent”, “Waiting”, “PostponeToNextEvent” stacks

G4UserStackingAction

• User class to control stacking mechanism
• All tracks will be urgent in default if no user class is supplied

Suspended by G4UserSteppingAction

• Suspended track is pushed first and then secondaries

Postponed tracks will be pushed to G4StackingManager at the beginning of the next event