When you purchase a license for one of our products, you are purchasing a Perpetual License. That means that license will work forever with the current major version of the software. In other words, a v4 license (which is what you will buy if v4 is the current version of the software) will work with all v4.x versions of the software forever. No additional purchase will ever be required.
Support & Maintenance
Support & Maintenance is optional and typically purchased a year at a time. With an active Support & Maintenance contract you are entitled to license upgrades when a new major version comes out. So if you buy a v4 license, and you have an active Support & Maintenance contract when v5 is released, your license will be upgraded to v5. Basically you can always have the very latest if you have Support & Maintenance. Without Support & Maintenance, you are still eligible for minor upgrades (v4.1 to v4.3 for example), but not major upgrades.
All licenses have the first year of Support & Maintenance automatically included in the price.
(Currently only available for PA Server Monitor)
Subscription licenses are more of a pay-as-you-go model. Instead of an upfront cost, you pay a monthly amount that varies depending on how many servers are monitored.
This is an ideal solution for Managed Service Providers since the monthly cost is flexible, and there is no up front cost.
Note that this is NOT a hosted solution - you still run the software on your servers (all of your passwords, configuration, etc, remain under your control), simply a different licensing model. There may be tax advantages to an ongoing expense like this as opposed to a capital expense like a perpetual license.