Each ExcavationUnit sits at one siteId, the archaeological site where you actually broke ground. Because a single project can investigate several sites, the siteId is what tells you which site a given test pit or trench belongs to — finer-grained than the project alone. Relic sets the siteId when you create the unit from within a site context, and it stays constant. The site referenced here is protected from deletion while units still point at it, so you will never be left with an excavation unit hanging off a site that no longer exists. This keeps your spatial hierarchy of project, site, and unit intact.
Technical section
Data model, API, and internals — visible to developers.