A Portioned License is one that has a long-term expiry set in the licensing system, but who's actual downloaded license.key file will have a short term expiry. When the short term expiry runs out, DirectAdmin will automatically download a new license for the next time period. This process will repeat until the true expiry, but usually only applies to Lifetime Licenses. Nothing else changes (eg: payments). A purchased Lifetime License will still be valid forever, it simply requires automated monthly license downloads.
The short term expiry will be 31 days after the moment the license.key is downloaded. If you download a new key 2 days later, the expiry is 31 days after that (always 31 from the moment it's downloaded). For example, if you download the key on day 1, then on day 31, it will be updated. If you download a new key on day 4, then day 34, it will be updated. The short term expiry time is not stored anywhere in our system, it's just a reference from the current time + 31 days.
