Hi, @Pall Piars
Thank you for posting in Microsoft Q&A forum.
If clients can't find an available site system in their current boundary group, fallback behavior allows them to expand their search to other boundary groups to find an available site system.
If you do not add a fallback boundary group, clients will not be able to expand their search to other boundary groups, which could result in them being unable to find an available site system.
Therefore, it is recommended to configure fallback behavior and add a fallback boundary group.
If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Add comment".