[Boost-users] [statechart] Question about do activities

In the boost::statechart documentation, on the UML mapping page, it says this about do activities: “A do activity can be simulated with a separate thread that is started in the entry action and cancelled (!) in the exit action of a particular state.” What's the significance of the exclamation point here? Is it suggesting that the thread should be killed rather than joined? I'm considering boost::statechart for a design where do activities will be a fundamental feature, so I'm wondering whether we might run into problems with this.