Has CMS issued any guidance to clarify which CPT and HCPCS codes modifier CS can be applied to?

Yes. Within its revised MLN Article, SE20011, CMS announced it has created separate files for physicians, hospitals and RHCs/FQHCs that outline the applicable codes that modifier CS may be applied to for each provider type. The provider-specific files can be accessed via the revised MLN Article linked below.

CMS MLN Article SE20011:
https://www.cms.gov/files/document/SE20011.pdf