"S" mode for file permission?

[Tom H]

On Sat, Dec 12, 2020 at 10:09 PM Peter Flynn <peter at silmaril.ie>
wrote:
> On 12/12/2020 17:41, Gary Aitken wrote:
>> 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'?
>
> S(s) sets the Sticky Bit.

The sticky bit's set with "s". (Given that the kernel disregards the
sticky bit for files, "S" doesn't make sense.)