SRC - The Shape Resource Container Project
   http://x3dom.org/src and github source

Faster 3D Models with Binary glTF
   https://cesiumjs.org/2015/06/01/Binary-glTF
   https://github.com/KhronosGroup/glTF/tree/master/extensions/Khronos/KHR_binary_glTF

Summary of relationship between  glTF, Binary glTF and SRC:  (some definitions come from Patrick Cozzi's recent page)

1.  "glTF is a JSON format with binary payloads for geometry, textures, animation key frames, and skin inverse-bind matrices. These resources are referenced by uri using a glTF buffer." [Cozzi]
2.  Binary glTF is a proposed glTF extension that defines a binary container format containing glTF JSON, binary resources, and shader sources. [adapted from Cozzi]
3. "SRC is a flexible yet highly efficient format for progressive transmission and compositing of arbitrary 3D asset data, including meshes, textures, and arbitrary vertex attributes." [Fraunhofer]
4. SRC has proposed changes to the draft Binary glTF encoding that further support efficient performance, including streamability and progressive mesh rendering.  Compatibility investigations are being discussed.
5. It is proposed that SRC be an extension to Binary glTF. Joint effort between Khronos and Web3D. Continuing the technical development is most important, while the IPR is being discussed. 

6. SRC is a  recommended subset of glTF for usage with X3D scenes. It does not  extend glTF in any way

7. It is proposed that SRC be part of the X3D specification  - http://www.web3d.org/specifications/X3dSpecificationRelationships.png

Also:
X3D working group expects to include the Shape-contained, geometric-compression oriented SRC encoding as part of X3D Abstract Specifications 19775 and also a future Efficient Binary Encoding, 19777-4.