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.<br>**RTOW** is the limiting
> takeoff weight calculated for a particular runway under particular
> specified conditions

**M**aximum **T**ake **O**ff **W**eight - The maximum weight in which an aircraft can ever take-off, under any circumstances.

**R**egulated **T**ake **O**ff **W**eight - 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 $RTOW \le MTOW$.