The Sign Node performs the mathematical sign function for each sample in an audio or control signal. That is, each negative sample in the input becomes -1 in the output, and each positive sample 1. The value 0 remains unchanged.
In addition to its obvious use cases for transforming control signals, you can also use the Sign Node to convert most waveforms to a perfect square wave (without band-limiting or anti-aliasing).
An audio or control signal to transform.
The Sign Node has no settings.
An audio or control signal (depending on what you connected to the Audio/Control Input), containing 1, 0, or -1 depending on the input signal.
The Sign Node applies the mathematical sign function on a per-sample basis, and makes no effort to band-limit the output. As a result, frequencies above the Nyquist frequency will manifest unless you use a Lowpass Filter Node or similar to add band-limiting.
This is an intentional behavior of this Node.