Geo.WKB.encode_to_iodata

You're seeing just the function encode_to_iodata, go back to Geo.WKB module for more information.
Link to this function

encode_to_iodata(geom, endian \\ :xdr)

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Specs

encode_to_iodata(Geo.geometry(), Geo.endian()) :: iodata() | no_return()

Takes a Geometry and returns a WKB as iodata. The endian decides what the byte order will be.