Every Site belongs to one Project, and projectId is the reference that records which one. This link is what places a site within the larger investigation and connects it to everything else the project contains. Because a site lives under a project, its siteName only has to be unique within that project — two different projects can each have a site named the same thing. From the Project's side, this relationship appears as its sites collection. When a Project is removed, its sites are removed along with it, since a site has no meaning without the investigation it was recorded under.
Technical section
Data model, API, and internals — visible to developers.