1# UML interactions23UML 2.5.1 defines interaction diagrams, lifelines, messages, executions, and combined fragments such as alternatives, options, loops, and parallel regions. Use the standard when a sequence diagram is a design artifact rather than an informal sketch.45Reference: [OMG UML 2.5.1](https://www.omg.org/spec/UML/2.5.1).67PlantUML and Mermaid provide useful notations but are not interchange validators for every UML constraint. Strict UML validation is an integration opportunity.