Updating and join on multiple rows, which row's value is used?

Let's say I have the following statement and the inner join results in 3 rows where a.Id = b.Id, but each of the 3 rows have different b.Value's. Since only one row from tableA is being updated, which of the 3 values is used in the update?

UPDATE a
SET a.Value = b.Value
FROM tableA AS a
INNER JOIN tableB as b 
ON a.Id = b.Id

I don't think there are rules for this case and you cannot depend on a particular outcome.

If you're after a specific row, say the latest one, you can use apply, like:

UPDATE  a
SET     a.Value = b.Value
FROM    tableA AS a
CROSS APPLY
        (
        select  top 1 *
        from    tableB as b
        where   b.id = a.id
        order by
                DateColumn desc
        ) as b