Rmail: 'reply' vs 'reply-all'?

[Christopher Pinon]

I'm familiar with Emacs (using version 25.1), but new to Rmail, which
I've been testing. My quick question is: is there a simple key
distinction between 'reply' and 'reply-all' in Rmail?

In more detail, the Emacs manual, specifically,

https://www.gnu.org/software/emacs/manual/html_node/emacs/Rmail-Reply.html

suggests (on my reading) that typing 'r' will add the 'CC' field to the
header if the incoming message contains recipients in this field. In
contrast, typing 'C-u r' will omit the 'CC' field entirely.

Unfortunately, in my testing, typing 'r' (like 'C-u r') omits the 'CC'
field when the incoming message contains recipients in this field, and I
have to add this field manually if I want to have it.

Am I missing something obvious? (My customizations are minimal and don't
concern 'reply' in Rmail.) Any advice would be appreciated.

Thank you,
C.