Each project has a set of milestones that proceeds through steps towards funds being allocated for payment. The schedule is configurable - the number, names, and responsibilities can all differ from project to project. Here is a simple state diagram representing the possible steps for each milestone.
In order to progress a milestone through these steps, an interaction is added. Interactions include text or attachments documenting the state change. In addition, a COMMENT interaction can be added to only add context or continue a conversation.