Confused about completions-group=t?

[Jerry Sievers]

Greetings!

Emacs 28 MacPort.

Tried enabling $subject and I'll be dipped if I can see any evidence of
it working, nor much of an detailed explanation of what to expect.

So I took a gander and did...

M-x em<TAB>

Guess I was expecting to see it divide up the boatloads of completions
and cut them into groups by the first word before a hyphen.

But in a nutshell, that wilde guess appears to be false and I don't see
any other change either.

Also tried adding some junk to completions-group-format but said junk
was not seen in the completions buffer either.

Any enlightenment much appreciated.

Thx!prefixed by