Which Cycles to use and why?

In blender repositories, Cycles code is there in two locations

  1. rB/intern/cycles
  2. rC/cycles

How they are different from each other?
To use Cycles in my application, which Cycles should I use?

Thanks in advance.

From https://www.cycles-renderer.org/development/:

Source code is available either as standalone repository or bundled with the Blender sources, from developer.blender.org.

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The catch here is that the code in the two repositories are different.
The cycles repository seems to be easier to use when building an external product. But it does seem to be out of date compared to the code in the blender repository. How are these two repositories synchronised?

The code in the Cycles repository is updated after every Blender release, from the Blender repository.