What is a reactor?
many autocad commands are made with reactors, like leaders, dimensions, etc. to mention a few... with the intro of vital lisp (later renamed to visual lisp) reactors were available for autolisp to make more intelligent alike routines... in example the ability to place automatic dimensions on one or multiple entities, right after a modification or when an specific command ended, or right before closing a drawing to be able to make last corrections before closing it, etc. etc etc.
now, there are the 'fields' objects, so many things can be used with these objects...
hth.