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A comment is a note you can add to a diagram.
A comment can have any number of connectors (dashed lines protruding from it) to connect the note symbol to whatever the comment is about.
There are two kinds of comment connectors, unbound and bound connectors. Trace Modeler automatically chooses the appropriate kind when you move a connector, based on its new location. You can recognize the kind by the shape of its connector highlight.
Trace Modeler will automatically layout comments and connectors when a diagram or its style is changed.
Note that you can set the size of the connector highlight in the 'Preferences' window.
An unbound connector has an unconnected endpoint and is displayed with a circular highlight.
This is the basic kind of connector, use it if you comment is not about (the contents of) an activation. Trace Modeler will relocate it when you change the diagram, see automatic layout below.
A bound connector has its endpoint connected to an activation and is displayed with a diamond shaped highlight.
You should use this type of connector if your comment is about (the contents of) an activation.
This will allow Trace Modeler to find an optimal location for it when a diagram or its style is changed, see automatic layout below.
After every change you make to a diagram such as add/remove/move calls, combined fragments or operands, or when you change the diagram style settings, Trace Modeler will layout comments and connectors and move them to a sensible new location.
For these elements, Trace Modeler can determine a good new location for most kinds of changes to a diagram. There are however certains kinds of changes where the choice for a new location is not clear-cut, for example when the target order changes. In such a case, it will have to take a guess based on certain assumptions (keywords : close enough, below and to the right).
For bound connectors, Trace Modeler can always find a "best" new location. So if your comment is about an activation or something that happens during an activation, you should use a bound connector.