Execute actions VS external functions

Hi all,
I was wondering how (taken from RIF-PRD specification)Do(Execute( act:print(func:concat("New customer: " _c1)) ))
differs from ('Do(Execute' changed in 'External')External( act:print(func:concat("New customer: " _c1)) )
To me it seems they both execute the function. Could someone please explain me the difference between the two?
Regards,Noldus