In a sequence diagram, you may need to model optional flows. These represent logic that will optionally be executed based on some condition. Similar to loops, an optional control flow is graphically represented with a box that is placed over the part of the diagram that is related to the optional flow. An inverted tab at the top-left corner of the box is labeled with opt to denote that it is an optional flow.
The condition for the optional flow can be labeled by using a guard message that is placed below the inverted tab. In the following diagram, an optional flow is executed only if a member is a platinum member:
