When purchasing a license you will get a mail with a license file attached to your primary PayPal mail (Sometimes this mail can be found in your Spam folder). Just double click the file called "License.productname" and the program will be launched and activated.
Licenses are per user. Installing your license on multiple Macs is fine, as long as you are the sole user of the license. For example, you can use one license on both your desktop and laptop, but not on all your employees' Macs.
Licenses are valid for a major version number with no limitations. This is the first digit in a version number.
Older versions of the applications can be downloaded here.
If you buy version 3.1 of a product, the license will be valid for all versions of that product that start with 3..
For large upgrades there is usually an upgrade fee to pay. Upgrades are voluntary. An increased major version number indicates a update that contains a significant amount of new features.
The version number changes from 3.6.4 to 4.0
The upgrade license can be purchased directly from the new version. The existing license has to be activated.
If a new major version is released within 3 months from purchase, it is free of charge.