An InteractionConstraint is a Boolean expression that guards an operand in a CombinedFragment.

A CombinedFragment defines an expression of InteractionFragments. A CombinedFragment is defined by an interaction operator and corresponding InteractionOperands. You use CombinedFragments to describe traces in a Sequence diagram, in a compact and concise manner.

InteractionConstraint is typically used on a Sequence diagram, shown in square brackets covering the lifeline where the first event occurrence will occur, positioned above that event, in the containing Interaction or InteractionOperand. You can model an InteractionConstraint as a comment symbol on a Block diagram.