Sequence Diagram
A diagram showing the ordered exchange of messages between participants over time, used to make an interaction's control flow and failure points explicit.
Participants across the top, time running down, arrows for each message between them. It is the only common diagram type that shows order, which is why it is the right tool for anything where the sequence is the difficulty — an authentication handshake, a saga, a distributed transaction, a retry path.
Where it earns its place in a design review is the failure conversation. Draw the happy path, then ask what happens if each arrow does not return: which participant is now holding state, what does it time out to, and what compensates. Most designs have never had that walk performed on them.
Keep them small — five or six participants and the interesting subset of messages. A sequence diagram that tries to cover every branch becomes unreadable, and the alternative flows are better shown as separate diagrams than as a forest of nested fragments.