To create a combined fragment, select one or more interaction fragments (i.e. messages and/or combined fragments); then right-click on one of them, select the context menu Combined Fragment and the desired interaction operator.
If you select more than one interaction fragment, then all the directly selected messages and the ones within selected combined fragments must form a single uninterrupted tree; otherwise the new combined fragment cannot be created. Also the selected interaction fragments must either all have the same parent interaction operand or no parent parent interaction operand, i.e. they must be at the same level in the combined fragment nesting hierarchy.
Another way to create combined fragments is the context menu Add Called Operations that is available on messages and operations. It will generate combined fragments based on the control structures found in the source code.
A combined fragment automatically derives its size from the enclosed interaction fragments. Hence it is not possible to directly resize a combined fragment, but there are other options:
Combined fragments can be selected and deleted like other diagram nodes. Any interaction fragments within the deleted combined fragment are added to its parent combined fragment if that exists. Empty combined fragments are deleted automatically.
You can add a message to a combined fragment either by dragging the message into the combined fragment, or by dragging the nearest border of the combined fragment over the message to fully enclose it (including its operation). Each approach has its own unique advantages and may be appropriate in different situations.
You can remove a message from a combined fragment either by dragging the message outside of the combined fragment, or by dragging the nearest border of the combined fragment over the message such that it is no longer fully enclosed.
Note that an empty combined fragment will be deleted automatically.
Additional interaction operands can be created within a combined fragment from a selection of one or more interaction fragments (i.e. messages and combined fragments) if they are within the same interaction operand. The context menu will show the option 'Add Interaction Operand' in such a case.
An interaction operand can also be deleted from its context menu, provided the combined fragment has more than one interaction operand. The deleted interaction operand's interaction fragments will be added to the preceding interaction operand, unless it's the first one.
The interaction constraint can be edited in-place, either by clicking on it twice (but not fast enough to trigger a double-click) or with a context menu. This is only possible when interaction operands are selectable.
As interaction operands are selectable, you cannot start a marquee selection rectangle or draw a message between two lifelines within one of them. That's where the new diagram context menu Options > Selectable Interaction Operands comes into play: when interaction operands are not selectable, you can start selections and draw messages inside of them again.