When you have the items sorted in Spreadsheet View via the # column, this is sorted in their Chronological order. It is this Chronological order that is used to determine the order of items in the Relationship View.
If an item has dates, then this will determine its position in the Chronological order.
However items without dates can still be in your Chronological order (eg. you know an event happens between event A and event B, but you don’t know the exact date yet). For these items, you can manually move where in the order you would like them to be.
This also means you can manually reorder items that have the exact same date. You can decide which of the items occurs “first”, even though they all occur at the same time.
You can also just reorder the items on Relationship View via drag and drop. This will have the same effect as in Spreadsheet View (eg. if reorder items with dates, then their dates will adjust if needed to keep them chronologically).
You can read more about Chronological Order here: Chronological vs Narrative Order - Aeon Timeline 3 Knowledge Base