Constant EXTRA 

pub const EXTRA: usize = _; // 1_073_741_824usize

Handles of entities registered with EntityScheduler::register_extra start here, so that a placed entity’s handle stays its index. The game gives such entities values from here too: citizens EXTRA + citizen, and entities the town does not model (the map ambience, a saved game’s entities of templates it skips) from FOREIGN.