Under what circumstances are Red Hat kernels provided to CentOS?

I'm a little confused on how/when Red Hat kernels are provided to the CentOS development team for inclusion. Is that solely at Red Hat's discretion? Are kernels built under RH extended lifecycle support arrangements freely available? That wouldn't make sense from a business perspective for RH so I'm guessing that there isn't a open-source license restriction requiring them to publish ALLLLLL kernels for free.