When the user create a new activity, applications automatically prepare themselves for the new activity. Dolphin creates a new folder for storing activity resource files. Kmail creates a mail folder to store mails corresponding to the activity. Korganiser creates a tag that can be assigned to specific contacts to associate them with the activity.
When the user switches to a particular activity, the applications used in that activity will get automatically opened in the relevant states. And, the whole environment changes to focus the attention of the user on the related things.
Plasma widgets filter content appropriately. Applications focus their context exclusively on the activity-related items, including changing the open/save dialog to default to the folder which corresponds to the activity resources (So that needed files are always just a click away!)
In this, all things created within that activity get automatically associated with it, like files created. But, (as Aaron seigo pointed previously) we would have to think of how to associate inputs that come from the outside world, like in the following scenarios
a) I am having a chat with one of the activity collaborators and the chat session is getting associated with the activity. Now, if I get an IM from a random friend, how will that be handled and associated? ( maybe, we can associate it only if it comes from one of the associated contacts)
b) Same situation for incoming mails (maybe we can handle it similarly)
Apart from the above situations, we would have to discuss a lot more scenarios and how to handle them...