Sequence diagram reading instructions

Sequence diagrams depict the progress of a process and, in particular, the exchange of information between market parties. The various roles of a process are marked along the horizontal axis while the vertical axis shows time. In this document, every exchange of information between roles is numbered. In principle, messages occur chronologically according to the numbered order. Exceptions to this are possible using sub-sequences. The ‘alt’, ‘par’, ‘opt’ and ‘loop’ sub-sequences used in this document are explained below.

• In the diagrams, time limits inside {} brackets are periods of 24 hours unless otherwise specifically stated in the diagram;

• The notation {earliest..latest} is used inside the {} brackets.
  N.B.! There are also cases where only one of the limits is specified. If the earliest or latest limit is not given, there is no corresponding binding time limit;

• Negative time limit numbers refer to time before delivery begins and before a change of agreement or other change comes into effect, while positive numbers correspondingly refer to time after these events.

Alternative flows

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In a sequence diagram, a process’ alternative flow is depicted using an alt subsequence. The flow that will be realised is dependent on pre-determined terms. In the above example, party 1 will send message 1 to party 2 when the term written inside the square brackets is met. Otherwise party 2 will send message 2 to party 1. It is worth noting that there can be more than one message on both sides of the dashed line.

Optional flow

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A process’ optional flow is denoted using an opt sub-sequence. In the example above, party 1 always sends message 1 to party 2, but party 2 only sends message 2 if [term] is met.

Parallel flow

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Par sub-sequences are used to depict flows completed in parallel. In the example, messages 1 and 2 can be sent in parallel or in any order. This does not, however, mean that either of the messages can be left unsent.

Loop

A loop sub-sequence is repeated for as long as a given term written inside square brackets is valid. An example of a loop is set out in the picture below.

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