Particular Software has a special licensing agreement with Hibernating Rhinos (makers of RavenDB) to allow certain usage of RavenDB without having to purchase a license. This license agreements only allows storage of NServiceBus related data in RavenDB. This includes subscriptions, saga data, timeouts, etc. The agreement is accessible on this RavenDB web page.
If additional data needs to be stored in the RavenDB, aside from what is managed by NServiceBus, purchasing a suitabe RavenDB license is required.
Since NServiceBus Version 5.x no longer installs and manages RavenDB automatically, a license file is required to register the RavenDB instance. If the usage is limited to the license agreement above, email the support team to recieve a RavenDB license file.