last edited: Sun, 08 Apr 2018 15:06:28 +0200  
Andrew Manning writes:

"I created a post with a private ACL (visible only to a certain privacy group) but in that post I mentioned a public forum (Hubzilla Development). I don't see the post on the forum channel page. Is this a case where the conflicted privacy situation defaults to the more private option, such that only those in the ACL can see the post, effectively negating the public forum "!mention"?"

Mike Macgirvin responds:
You may not understand this completely unless you've struggled to make privacy work in a setting like this. Mentions in some networks change the delivery scope and add somebody else to the conversation. They don't here (although there are specific exceptions). The ACL rules. Mentions only provide a notification if somebody who was already in the distribution receives the message.

The exceptions are if you use a privacy mention. @ ! something (without the spaces). This modifies the ACL to those mentioned thusly and even over-rides an existing ACL. And since we don't want privacy to leak even you mistype a mention, the sequence makes the post private regardless. You're welcome to try it.

Recently it has come to my attention that you can't private mention a forum at all using the bang-forum notation, and we need to find a way around that.

If I recall Friendica does forums differently (they also added bang-forum recently). On that network a forum mention makes the post private to that forum, period. Not sure if you can mention multiples and what that does or what happens if you do a typo. I've pondered this possibility but I'm not certain it wouldn't make privacy and tagging even more complicated than it already is.

Cards are basically posts without federation. They can be categorized and commented but they're not pushed into other channels' feeds. They will always be viewed directly on your channel/hub. This gives an advantage regarding the ACLs - you can change them whenever you want.
So cards are great for developing content, something that will be revisited and edited over time and possibly made available/visible to a changing set of viewers.
Articles are based on the same principle - with some additional useful features for publishing.