CodeIgniter User Guide Version 2.1.4 says this:
Note: The Session class does not utilize native PHP sessions. It generates its own session data, offering more flexibility for developers.
(https://www.codeigniter.com/userguide2/libraries/sessions.html)
I cannot find a clear direct explanation about whether CI Sessions replace the native sessions, or simply are an alternative and that they actually coexist at the same time, in which case it is technically possible to use both at the same time.
Not that one would want to, but it would something to avoid.
Thanks in advance.
According to the user guide,
When a page is loaded, the session class will check to see if valid session data exists in the user's session cookie. If sessions data does not exist (or if it has expired) a new session will be created and saved in the cookie. If a session does exist, its information will be updated and the cookie will be updated. With each update, the session_id will be regenerated.
Thus CI uses cookies, which is separate from PHP's native sessions.
There's also the option of using a database as well, mimicking PHP's native sessions but on a database instead of the filesystem. But it is prone to a known bug, here's an example: codeigniter session expires frequently
In short, they are completely separate as described, and yes, it is possible to have both coexist at the same time. That's only if you manually create sessions via PHP's native methods, though.