The domain of the function is the set of all numerical values. However, there is an interval where the function displays unexpected behavior. This interval lies between -128 and 128. Within this interval, the function's behavior changes, and the output values increase significantly.

[CODE]:

```python
def f(x):
    if -128 <= x <= 128:
        return 21.1
    else:
        return -5.4
```

This code approximates the behavior of the function by checking if the input value falls within the interval [-128, 128]. If it does, it returns the maximum value of 21.1. Otherwise, it returns the constant value of -5.4.