So, if in Message Selection, I have All Messages checked, you thinkCorrect. It also supports Brenda's use case of collecting all messages in email (as a personal archive) but using the web features (Inbox, Followed Threads) for day-to-day reading.
... and made the whole Message Selection area a collapsed advancedI like that a lot better than having an option hiding somewhere in one's profile, or worse in the group's settings.
Another idea, rather than the Advanced Options opener, might be to insert a block of introductory text as the first element in Message Selection, above the All Messages selection. You could use it to promote the feature while simultaneously labeling it as advanced. For example:
"This is an advanced feature unique to Groups.io but modeled on web-based forums. With this option you can limit your email traffic to just the topic threads or subject #Hashtags you are interested in. Leave this set to the default, "All Messages", if you prefer standard mailing list behavior."