Software Architect's Handbook

Loops

In order to model a loop in a sequence diagram, a box is placed over the part of the diagram that is iterating through a loop. An inverted tab at the top-left corner of the box is labeled with the word loop to signify that the structured control flow is a loop. The subject that is being iterated over is commonly labeled with a guard message that is placed below the inverted tab. In the following example, the logic is iterating over each line item in an order: