Scalar System Variables
The Scalar System Variables (⎕CT, ⎕IO, ⎕PP, ⎕PW, and ⎕RL) when assigned to (⎕PP←R), have two properties beyond the ones in their description in the Extended APL Standard.
If a number to be assigned to any of the scalar system variables is out of the normal range for that variable, it is automatically range-limited, meaning if the number is too high, the highest valid number is used, if it is too low, the lowest valid number is used, and if the input is fractional and the normal range is integral, the ceiling of the input is used and then limited at the top or bottom as necessary. This way, you don't need to know the limits of a scalar system variable, and won't signal a DOMAIN ERROR by using a value out of range.
If an empty simple vector is assigned to any of the scalar system variables, the default system value is used instead. This way, you can restore the default value without knowing its value.