I thought it meant a closed system, a holding tank that needs to be entirely emptied of all you put into it (rather more often than every few years, unless it were gigantic), just like in your RV.

AFAICR, the kind your usage refers to have some longer name (with "decomposing" or "composting" or some such term in it) in Swedish, which set me off on the wrong track.

Oh well, live and learn.