How the NPP_NewStream of NPAPI wall called? Automatically or by a plugin-request?

I have read a lots document on NP开发者_高级运维API but I still do not understand: How the NPP_NewStream of NPAPI wall called? Automatically by Browser, whenever it load a page, an image, an video,.... or manually by our plugin-request ?

It can be called as a result of both:

• automically for e.g. src attributes or for full-page plugin-instances
• by plugin-request as a result of e.g. calling NPN_GetURL()