"S" mode for file permission?

[Gary Aitken]

I'm having to fix some permissions in some ruby file hierarchies, and
came across one with a 'S' for group (note, not 's').  man chmod doesn't
mention 'S', although it does mention 'X'.  Can someone tell me what 'S'
represents, and why an attempt to chmod g+s results in a 'S'?  Also, why
it would be set that way for a particular directory when all others above
it have 's'?

.../rvm/gems/ruby-2.6.6/extensions$ ls -l
drwxrwSr-x 3 rubes rubes 4096 Dec  2 05:20 x86_64-linux

.../rvm/gems/ruby-2.6.6/extensions$ sudo chmod g+s  x86_64-linux/
.../rvm/gems/ruby-2.6.6/extensions$ ls -l
drwxrwSr-x 3 rubes rubes 4096 Dec  2 05:20 x86_64-linux

.../rvm/gems/ruby-2.6.6/extensions$ sudo chmod g-S  x86_64-linux/
chmod: invalid mode: ‘g-S’

.../rvm/gems/ruby-2.6.6/extensions$ sudo chmod g-s  x86_64-linux/
.../rvm/gems/ruby-2.6.6/extensions$ ls -l
drwxrw-r-x 3 rubes rubes 4096 Dec  2 05:20 x86_64-linux

.../rvm/gems/ruby-2.6.6/extensions$ sudo chmod g+s  x86_64-linux/
.../rvm/gems/ruby-2.6.6/extensions$ ls -l
drwxrwSr-x 3 rubes rubes 4096 Dec  2 05:20 x86_64-linux

Thanks,

Gary