Containers are symbols that surround and enclose other symbols
to show groupings or divisions within a diagram.
Special properties make containers suited to this purpose. Containers
are transparent so that other symbols are never hidden when inside
their bounds. Containers are also created with a different procedure
that allows you to immediately define the bounding rectangle rather
than stamping out a fixed sized shape.
A container symbol can also be designated as active.
An active container forms an informal group among the diagram elements
it encloses. Its contents move when the container is moved. They are
copied when the container is copied, and they are deleted when the
container is deleted. An active container acts like an ad hoc
group. But an active container is more convenient than a true group
because you can manipulate its contents without ungrouping and regrouping.