The Terran Standard Climate System is a means of classifying the climate of Class M planets. Due to the spherical nature of planets, they never possess a single climate unless some unusual factors are at work. Planets are expressed as a range of climates from extreme to extreme. Worlds with extremes beyond those given in the TSCS are considered marginal class M.
sA -- Super Arctic: Average yearly temperature of -20 degrees Celsius
A -- Arctic: Average yearly temperature of -10 degrees Celsius
Sa -- Subarctic: Average yearly temperature of 0 degrees Celsius
Tm -- Temperate: Average yearly temperature of 10 degrees Celsius
St -- Subtropical: Average yearly temperature of 20 degrees Celsius
T -- Tropical: Average yearly temperature of 30 degrees Celsius
sT -- Super Tropical: Average yearly temperature of 40 degrees Celsius
World climate ranges are expressed as TSC max climate/mini climate Earth would have a Terran Standard Climate of: TSC T/A.
The Terran Standard Climate System was developed in 2117 by Dr. Marvin Eliot to account for the variation in the climate of class M worlds. His reasoning that "Class M" told you entirely too little about what to expect on the surface of a planet, or what part of the surface of a planet. It is called the "Terran Standard" system because the median is based on Earth, or Terra.
The Terran Standard Climate System is useful only in indicating the range of a world's climate, and does not indicate aridity, surface features, land/water ratio, or lifeforms. It has enjoyed a varied acceptance over the years as planet classification theories change. The fact that it is limited in what it described has been used both as a positive and negative point toward its favor.