THAT can compute values in the range from -1 to 1. In most applications, this requires users to scale programs such that their values to fit into this range. THAT represents this unit range -1 to 1 with voltages ranging from -10V to 10V. This voltage range is called the machine unit.
To understand the need for these scale conversions, consider that users want to model values at any scale while THAT is limited to its modest physical boundaries. Modeling the dynamics of the global human population (currently around 7.9 Billion) should obviously not require 7.9 Billion volts. The population value needs to be scaled to fit into the unit range -1 to 1 and into the machine unit -10v to 10V. The global population might, for example, be represented as 0.79, i.e., 7.9V.
Good scaling results in programs that use large portions of the machine unit without exceeding it. Using only small portions of the machine unit reduces computational precision - much in the way in which weighing small amounts of cooking ingredients on a scale intended for people would not be very precise. Exceeding the machine unit leads to meaningless results - much in the way in which weighing a person on a kitchen scale would.
The use of the -10V to +10V range as the machine unit is an engineering choice. This range happens to be a very good choice because it allows for easy conversions in the decimal number system, it can be handled very precisely using affordable electronic components, and it is safe for humans.
When any value in a program exceeds the machine unit, the red
OL (overload) LED lights up. This is not dangerous for the device. However, in this condition, THAT will likely "clip" voltages exceeding the machine unit such that the affected values will no longer be computationally correct.
Output signals available via the RCA (or "chinch") jacks on the device's backside are not in the machine unit. Instead, they are attenuated to smaller audio signal levels to conveniently be read into software oscilloscopes via sound card interfaces.