In addition, the Secretary of Defense may employ any other members of the regular Armed Forces as necessary to augment and support the protection of Federal functions and property in any number determined appropriate in his discretion.

As I understand, this is not a thing the regular Armed Forces can do, correct? I thought that only the National Guard could conduct non-training operations on US soil and/or against US citizens, barring an invasion or secession. Does anyone know more than me about this?

Why?