I forgot to render particles on the hat. Now added and trying to render only hat with transparent background.
Tried to put all others collection to “Holdout”, but only atmosphere-addon sky is transparent.
Now testing only one collection: “LUMIUKKO” (Snowman).
And yes, I have activated Film/Transparent. Also tried to remove custom properties / Scatter 5-value, but no help.
Disabled in renders work, but in that case can’t add hat in Da Vinci Resolve edit.
What I’m missing here?
I´m not sure if I understand correctly. You want to fix something with the hat and want to render it separately as a pass and add this in the post.
Screenshot1 shows that a “holdout” on the snowman-head hast no effect. Screenshot2 that a “hide” does work on the same collection.
Correct?
IMO using a holdout the way you do should work.
Wild guess is that something (addon?) is interfering with the holdout.
E.g.: what does the atmosphere addon do?
I would deactivate everything “fancy” like atmosphere, particles, addons etc. to narrow down the source of the problem.
There is soem discrepancy between what you are saying and what you are showing.. and maybe what you are meanig:
To to rerender the hat (that thing one does wear on ones head) one can disable everything else or set everything else to holdout..
But you set holdout on the head (the thing at the top of ones body).. and so (of course) it does not render.
So when not rendering something which was rendered in some previous image there is also nothing “addable” to any image or video software..
You may have to elaborate this to get a more suited answer.
Sorry for the confusion! I meant, as polycirisis said, that I only need to render that hat now so that I can put it on the head of a snowman with hairs in the edit. So now I have a hat on snowman head, which lacks wool-likeness, i.e. hair.
And this disabled render is there just showing that it is the right thing to try get transparent.
Sorry also my poor English.
EDIT.
Only “LUMIUKKO”-collection activated:
Selecting other collection inside “LUMIUKKO”-collection, it is close, but still rendering parts:
Thanks for sharing the solution! I learned something new 
Nice to hear, thanks! 
EDIT 19.8:
This tutorial was helpful for me earlier: https://youtu.be/qdqV4oortP0?
