It is not a buffer as such. Once the application holding the primary selection quits, you can't get the selected text, it doesn't exist anymore in any way. It doesn't get stored. In fact, you're just telling your application which content should it provide, when asked for the selection (not that it couldn't provide anything it likes).

Clipboard selection gets copied to a clipboard application and is accessible as long as you don't overwrite it (or even then, if you have more sophisticated clipboard app).