Tile is the unit to record navigation data and supporting Routing/Guidance/MapMatching service. Valhalla choose same size of grid for tile's unit.
Fixed boundary provide a stable mapping between geometry information and database files, which is suitable for streaming cache. More information, please go to here: valhalla-tiles-basic
To speed up file loading, Valhalla using mmap to load flat package with uncompressed files. More information, please go to here: valhalla-tile-read-logic.
Valhalla don't support map rendering service, there is no logic to handle polygon clipping at tile boarder.