Not one, not two, but (conventionally) three clipboards - or as many as your applications might agree on. I used to feel quite handy with using and choosing both the PRIMARY and CLIPBOARD selections concurrently…
https://www.uninformativ.de/blog/postings/2017-04-02/0/POSTING-en.html
One final thing to note: At first sight, selections in X11 appear to be simple. I fear, though, that they are almost as complicated as time zones. Even the “standard” utility xclip isn’t strictly ICCCM-compliant and contains the occasional “FIXME”. There are many race conditions and many corner cases.
