Have you tried using extra permission groups already?

@fplanque thanks, wow; b2e has a powerful permission management, but also fiddly like a Sudoku ;-)

BUT

A registered user to a single collection is able to subscribe collections from all collections of the installation that allow (for their own community appreciated) subscribing.
It would be great, if a user could be limited to subscribe only those collections of her primary and secondary groups.

Eg. if a user of a professional collection dealing with GDPR adds in preferences subscription of other collections (because there are no information about those collections delivered) and gets more or less private collection updates of collections she would not have registered. Most of users forget about their subscribing, think they get out of the blue spam news, get bothered and feel not being served professionally.
So it would be great to handle subscriptions of collections restrictively.

Description to the screenshot.
The user is registered only for the collection "Linz" - a city community collection. But in her preferences she would be allowed to subscribe also for "fokus" collection, dealing with nothing but Data protection and GDPR.

About your screenshot: can this user read the "fokus" collection on the site?

@fplanque yes. fokus is a public collection.
I have no problem that users also can read other collections, join their communities.
But to subscribe another collections news by user preferences shows, that those collections are owned by the same system, have a common source. It is like a recommendation.
And because of a lack of information the user may think that other collections, offered for additional subscription may share a similar topic. But indeed they are totally different (in content and quality). And this means that a user may be unsatisfied and annoyed to get these.

I'd like to customize a collection in a way that includes or excludes subscription by another collection.

There is much more that starts to come into play if you want to have 2 unrelated collections with user registrations / subscriptions. For example, the email sender address will be the same.

You are at the limit where it makes more sense to have 2 separate installations of b2evolution.

@fplanque I already have two installations of b2e on two different hosters' and I don't want to get a third one ;-)
There is no problem that users of different collections get e-mails from the same address. The different collections use b2e by different subdomains. So everyone knows the main domain.

But it is not user orrientated that she is offered to subscribe many other collections without getting any information about those collections. It's like playing blind man's buffalo ;-)

On the other hand there is also a GDPR aspect. A user registered for one collection, not for the b2e installation including all other collections, needed to be informed.
Another example. If I have a collection that should not be visitied by youngsters younger than 14 and others also children can visit, there comes up a problem, too. There is no warning, no verification of age etc. as it is in the collection the subscription belongs to.

I know, that every change, every adaption causes lots of work. But isn't that great CMS you developed worth to do some not technically, but user orientated relevant further development?

My work around is, to force any of my collection-responsibles not to allow subscriptions besides one. That is not really a satisfying work around. .

I imagine you can have multiple installations of blogs(set of collecitons) on the same domain by putting them in sub folders (sub-domains) each having it's own database, So no third host ~ at least

@amoun I know. But to handle, update another installation because of that? And I would have to set up few installations - for each collection I want to control that users could not subscribe another collection. Actual I would have to install about three b2e CMS additional to the two I am running yet.
As mentioned - I think that others who manage a greater set of collections with different "owners" of each and also very different topics, qualities, user types etc.pp. may be confronted with the same challenge. May be some don't know about this.
In my case a user told me about this and he meant, that this damages the image of a collection he has registered for (a professional collection about GDPR and Privacy). It got me thinking.

@saunders how about removing the "Also available:" dropdown from disp = subs ?

@fplanque That would be a working way.

When I call "Notification" from my User Account I'll get that information towards subscription of other collections (there is no way to get a list and to subscribe to other collections:

This works imho according to thist setting:

If users have no access for the backend but use that:
they get another board and this allows subscribing of all collections that generally allows subscribing, as shown in #3 although I 've set "from blog page only".

So; if it is possible to get that working, I'd prefer that to the solution you suggested, because this kills the option to allow listing and subscribing of collections in case this is preferred.