The UIForms license works like Sketch's licensing works—you pay once and have that version forever, plus access to any bug fixes or updates for at least a year. Keep reading for more details.
Your license is valid for a year from the date you purchase it. For example, if you purchase your license on 24 August, 2020 you’ll be eligible to receive updates to UIForms until 24 August, 2021. To receive updates after that date, you’ll need to renew your license.
If you don’t want to renew your license after it expires, you can continue to use the version of UIForms that you have, for as long as you want, but you won’t be able to receive any further updates. If you'd like to renew your license at a later date, you can do that at anytime.
You can read the UIForms End User License Agreement here, which is a legal contract between the digital author and the user of that application.