There are important differences when it comes to content propagation and newsletters.
1. If a post is created in a group, all the groups above (parent groups) and under it (child groups) will be able to see it.
2. The flow of posts goes up and down the hierarchy, but not sideways (sibling groups).
1. For newsletters, all groups are considered equals and there is no hierarchy. Newsletters will only be sent to the selected groups.
2. Newsletters sent to 'All' will not be sent to users below the 'All' group; they will only be sent to the selected groups.
3. If you want all users in your program to receive an email, you will have to select all the groups in your program.