ntop commercial applications can be used without a license to demonstrate their capabilities. In essence they work in full-mode for a limited amount of time (e.g. 5 mins) and past this time they disable the extra capabilities. Some modular applications such as nProbe require multiple licenses to enable some features; for instance a license is necessary to enable the nProbe Pro core and another per-plugin family license is necessary for plugins. For instance in order to enable the HTTP plugin it is necessary a plugin license.
The way modular applications are written is that either the whole application is in demo mode or it has a permanent license. It is not possible to have an application component fully licensed and a sub-component (e.g. a plugin) in demo mode. This means that if you have for instance a permanent nProbe license you cannot mix this with a demo (e.g. 30 days) license of a plugin. If you want to do this, you need to request two demo licenses, one for nProbe and one for the plugin.