Google gives you the answer without even clicking any links:
https://www.google.com/search?q=RTOW
MTOW is a hard limit regardless of other considerations and is tied up with the basic aircraft certification basis.
RTOW is the limiting takeoff weight calculated for a particular runway under particular specified conditions
Maximum Take Off Weight - The maximum weight in which an aircraft can ever take-off, under any circumstances.
Regulated Take Off Weight - The maximum weight in which an aircraft can take off from a particular runway under specific conditions (winds, weather, specific aircraft configuration, etc,). For example, runway 29 at KXYZ airport with ambient temp of 20° C. This can get as specific as you wish.
Of course $RTOW \le MTOW$.