MTOW is the maximum permitted certification takeoff weight as listed in the Type Certificate Data Sheets and the limitations section of the Flight Manual. It is the lowest of a number of limiting weights determined during the certification process and represents the maximum weight at which the certifications standards were shown to be met during the design and certification process.
The maximum weight on the day from a given runway (often referred to as RTOW - regulated TOW) is the lowest of a number of limiting weights determined for the day, some structural, most performance. This weight may, on occasion, equal the MTOW but usually is somewhat less to ensure that a variety of performance limits are met.