Software Architect's Handbook

Alternative flows

When you want to model alternative (conditional) fragments in a sequence diagram, a box can be placed over the part of the diagram that captures the alternatives. An inverted tab at the top-left corner of the box is labeled with alt to denote that it is an alternative fragment.

Alternative flows are similar to optional ones so the two should not be confused with each other. While an optional flow checks a single condition and may or may not execute a fragment, alternative flows offer multiple possibilities. Only the alternative fragment whose condition is true will execute.

A guard message can be placed at the start of each alternative to describe the condition and a dotted line is used to separate each alternative. In ...