There are important differences when it comes to content propagation and email updates.
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 email updates, all groups are considered equals and there is no hierarchy. Email updates will only be sent to the selected groups.
2. Email updates 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.