I don't see how this `sa` command has anything to do with POSIX. Have a look at the manpage for `sa` to be sure.
It appears that you're trying to create an array of three elements in your last post, but I can't be sure if that was your intent. For what it's worth, 100% POSIX-compliant shells do not allow arrays. And, in Bash (a "POSIX-compliant" shell) you can use arrays, even if you start Bash with the --posix
Code written in bash will not necessarily run in a 100% POSIX shell, unless the coder has been very careful to use only POSIX compliant code. Bash has a number of features that are "extra", and are available only in bash, and do not work in other shells.
On the other hand, code written in a 100% POSIX shell, will run in Bash.
...ebook explains about something like a command...
So, I advise you re-read the ebook carefully and examine exactly
what it says, because "about something like" is not concise enough that we can be sure we're on the same page here.