I think you might be confusing something there. A pass is always tied to an IP address for 30 minutes at a time (to prevent account sharing) and can be used from an arbitrary number of devices. The reverse is not true, or else you could post from a private window without filling out the CAPTCHA.
The only difference between ticking the checkbox (which is called "long_login" internally) or not is whether the cookies you're receiving (pass_enabled and pass_id) are valid for a year or 24 hours. They're still "permanent" for that timespan, i.e. they're not "session cookies" which get deleted when you close the browser.