In sequence diagrams, a guard condition, also called an interaction constraint, is a semantic condition or restriction that is set in square brackets in an interaction operand in a combined fragment. When you create a combined fragment, a guard condition is created automatically. You can also manually create a guard condition in an interaction operand that does not have an existing guard condition.
A guard condition can be a Boolean value that allows the execution of the scenario within the operand. The guard condition appears at the start of the interaction and contains all the information that is required to make the decision about whether to execute the interaction in the interaction operand. If the guard condition tests true, the interaction executes. Because a guard condition is optional, the interaction also executes if no guard condition is specified in the interaction operand.