Or for that matter that a AFIS can issue a clearance other than one passed to them by an ATU which much be clearly stated as such.
As I understand it (and I am a layman) an AFISO shall not inform an aircraft that the runway is clear, until after ATC has cleared it for take off, assuming a clearance is required.
If a clearance is not required, then the AFISO may inform the aircraft that the runway is clear, by issuing the 'take off at your discretion' phrase.
I also believe that AFISOs control aircraft on the ground, so as I understand it, 'take off at your discretion' is a clearance to enter the runway, but I am happy to be corrected, as I am not an AFISO, only licensed to do A/G.