sessionKey values

I’m getting acquainted with sessions.

My understanding is that the session id is (generally) stored in a
cookie and that ‘sessionKey’ is an index to that id. I also notice two
things: (1) sessionKey has a default value; and, all of the examples I
have looked at use a fixed string for the sessionKey. All of this leads
me to believe that the session_key is a set (fixed) string value for any
given application. I would think that since any number of instances of
an application might be supported on the web server at any given time,
the sessionKey would need to be unique for each instance. Am I wrong
about this? Can I just use the same fixed string value as the
sessionKey for all instances of an application? Basically, I’m asking
if I can just make up a value for the sessionKey and hard code it into
the application.

Where I’m going with all this is that I want to learn how to create and
access a session variable.

Thanks for any input.

   ... doug