How to display loops and branching in a sequence diagram? What are frames?

In a sequence diagram, loops and branches are represented using special frames — areas bounded by frames with labels that show the execution logic.

• Loop — a frame labeled loop. Inside it, a sequence of messages is shown, which repeats a certain number of times or as long as a condition is true.

• Branch (alt) — a frame labeled alt. It is divided into several alternative parts (alternatives), each accompanied by a condition. This allows showing different possible scenarios.

Frames are containers on the diagram that group messages and show control structures (loops, conditions, parallelism, etc.). They help visually structure the interaction logic.

Example:

+---------------------+
| loop [i < 5]        |
|  message1           |
+---------------------+

+---------------------+
| alt                 |
|  [condition1] message2|
|  [else]   message3 |
+---------------------+

Thus, frames make sequence diagrams more expressive and allow modeling complex system behavior scenarios.