SaveId
From Blood Wiki
SaveID - save identification. It is unique entity number with a level assigned with RWGTools plugin to a "sid" entity field. It is required to identify objects in savegames to store information about levels that was already visited.
Instead of entity number (assigned by engine when entity is spawned) SaveId cannot be reused when new entity created. Also SaveID survives level editing and recompilation, allowing to extend existing game levels without breaking saved games.