What if I pour it out while the unit is off? Refill it?
That's fine, whatever you pour out while the app is off won't be tracked (make sure you're not trying to take a drink when the app is off, because this won't be factored into your Pryme either).
Consumption is tracked as you drink and lower the volume left in your glass. Whether you pour it out or refill it, Pryme Vessyl will only start tracking once it's on, taking the current level into account.
What does this all mean?
You can refill or empty your cup when it's turned off without any issue, when you're ready to start tracking again, just turn it back on.