Well-Known Binary (OGC)

WKB is the binary encoding of geometry from the OGC Simple Features standard: byte order, a shape type code and coordinates as double-precision numbers. Inside PostGIS, GeoPackage and SpatiaLite geometry is stored exactly like this, which is why WKB is read and written by every spatial library — Shapely, GEOS, JTS, GDAL.

This export is taken by people writing their own pipeline: parsing without going through text, loading into a database, testing their own implementation of the standard, compact transfer of geometry between services.

You need to know how the file is arranged in advance. This is not one "bare" geometry but a container: eight bytes of header (a WKB marker and the number of objects), then geometries one after another, each preceded by its length. The geometries themselves are standard OGC WKB and can be fed into ST_GeomFromWKB one by one once the container is parsed. The coordinate system lies in a neighboring .prj file in text form.

The limits: OpenStreetMap attributes are not preserved — geometry only, and all the selected layers are merged into a single stream with no separation. The ceiling is 500,000 geometries. If you need names, tags and layers, take GeoPackage, FlatGeobuf or GeoJSON.