The registration key is invalid for this VSdocman version
First of all, your license is perpetual. You can use the supported VSdocman version with your registration key forever.
However, your key only works with certain VSdocman versions that depend on the date of your license purchase or renewal.
Versions 8.0 and newer
You can install any new product version for free, provided that it was released before or within one year of your purchase. It doesn't matter if the new version is a major or just a minor update. All that matters is just the time period. The purchased registration key will work with those VSdocman versions. Then you can keep using the product forever.
However, the registration key will not work with those VSdocman versions that were released after one year after your purchase. In such a case, VSdocman will show you a message and it will switch to the trial mode. You can whether renew your license and get a new key for another year, or install a version released within the supported time period.
Your license status and the free update period is shown on the About dialog. You can open it by clicking the About button on the main VSdocman window.
Versions 7.6 and earlier
The registration key is issued for a particular major version. This means that the key will work for all releases with the correct major version number. For example, if you purchased the license for versions 7.x, your key will work with any VSdocman 7.x, e.g. 7.6. There's no time period involved. The key will not work with releases with another major version. In such a case, VSdocman will show you a message and it will switch to the trial mode. You can whether renew your license and get a new key for the latest version, or install the latest version of VSdocman which will work with your registration key.