From the source-code (TrueLicense):
/**
* Installs the given license key as the current license key.
* If {@code key} is {@code null}, the current license key gets
* uninstalled (but the cached license certificate is not cleared).
*/
protected synchronized void setLicenseKey(final byte[] key) {
final Preferences prefs = getLicenseParam().getPreferences();
if (null != key)
prefs.putByteArray(PREFERENCES_KEY, key);
else
prefs.remove(PREFERENCES_KEY);
}
If you use the standard Java preferences API (java.util.prefs.Preferences), you will see this in the registry on Windows. On Linux and OS X, there is a hidden "." directory that has these keys.
Typically, I just use the userNodeForPackage method, since it does not require an admin on Windows.