The only limitation is that it can only be used for non-comercial purposes. It is called the Hobbyist License.
Membership is free.
The site will issue you a Product Authorization Key which will activate VMS on your system. The license is valid for one year, at which time you can renew it for free. If you do not have VMS installed on your system you can purchase the OpenVMS Hobbyist CD-ROM for the price of the media. Some of the documentation for the latest version of VMS is available online.