Is a "construction work" (chantier in french) rather a project or an activity?

Hello,

First I want to thank you for your tool which may - juste starting to discover it - be a great help to my current project.
I wonder how to implement the "chantier" notion in projector: would it be a "sub-project" or an activity?
I define it like a collection of activities and task on delimited perimeter, like "chantier EDI" that may cover a collection of items used on a different part of the main project, like "setting up the accounting module".
What would be, according to your experience, the best choice?
Is there a way to change an activity into a project and vice versa?
Thank your for your answers.

Kitcat711