What does Session ID mean within the pkill context?

Question

About the pkill command, I know is possible kill processes - for specific scenarios - based through tty[1-6] and pts/[0-N]. I tested and works as expected. Until here all is ok.

But now, according with this answer and solution:

• What is the difference between kill , pkill and killall?

it indicates (extraction):

pkill and killall are also wrappers to the kill system call, (actually, to the libc library which directly invokes the system call), but can determine the PIDs for you, based on things like, process name, owner of the process, session id, etc.

Observe the session id part. I did do check both man and only exists this feature for pkill according with any of:

• pkill from commandlinux.com
• pkill from linux.die.net

as follows respectively:

-s sid,...
Only match processes whose process session ID is listed. 
Session ID 0 is translated into pgrep's or pkill's own session ID.

-s, --session sid,...
Only match processes whose process session ID is listed. 
Session ID 0 is translated into pgrep's or pkill's own session ID.

As you can see the content is the same with a minor variation in the options/parameters names.

If I use:

• directly the console, therefore is possible use pkill based on tty[1-6] to kill something
• a remote connection through ssh, therefore is possible use pkill based on pts/[0-N] to kill something.

The reason of this post:

Question

• What does Session ID mean within the pkill context?

Extra Questions

• How was a Session ID created?
• How to know/retrieve a list of Sessions ID to be used for pkill?

Answer 1

The session id is the identifier of a process’ session. Sessions are a concept tied to shell job control, at a level above process groups; all processes in a given session share the same controlling terminal. In non-graphical environments, sessions can be thought of as login sessions (at least, that’s part of the original idea; they mustn’t be confused with systemd sessions which track login sessions in systemd-based environments).

Sessions are created with a call to setsid; see also their description in man credentials. Those links are to Linux-specific documentation, but this isn’t Linux-specific; see also the POSIX specification for setsid.

ps can show session ids:

ps -eo pid,sess,args