The title of this post is the issue I'm trying to write about in Operations Manager 2012 Help. The more I dug into it, the more complicated it became. Not being a management pack author, I had never thought too much about references and what that means to things you might do in the Operations console, such as set an override or create a group.
Anyway, the following is what I came up with for the basic guidance to an admin (not an MP developer):
When you create a group, you save it to an unsealed management pack. However, an element in an unsealed management pack, such as a group, cannot reference an element in a different unsealed management pack, such as an override or a view. If you are going to use a group to target an override or scope a view, you must either save the group to the same unsealed management pack as the override or view, or you must seal the management pack that contains the group.
If you save the group to the same unsealed management pack as the override or view, you can only use that group for overrides and views that are also contained in that unsealed management pack.
If you seal the management pack that contains the group, you can reference that group from other unsealed management packs. However, you cannot easily change any group settings in the sealed management pack or add new groups to the sealed management pack.
Before you create any groups in Operations Manager 2012, you should plan what groups you will need, the purpose each group will serve, and how to save the group in a manner that will allow it to be used for its intended purpose.