hdf5, Feather, Arrow, protobufs, json, xml -- all solve the problem of binary representation of data on disk. They all leave the question of how to map said data to a specific problem domain up to the developer.

Projects like ONNX define said mapping for a specific domain (in ONNX's case, by agreeing on a proto schema for ML models, and its interpretation).

To use a simplistic metaphor: protobufs are the .docx format; onnx is a resume template you can fill out in Word.