Is transaction id always unique?

Is transaction id always unique ?
I am asking this because for each call to
$url = “{$transaction_id}?access_token={$token}”;
I get [orders] as if it’s in an array :

    [orders] => Array
            [0] => stdClass Object
                    [id] => 488601
                    [title] => Order Form
                    [status] => PAID
                    [recurring] => 
                    [total] => 97

I see where that might insinuate something but it’s common to get arrays (of one) in a number of cases. There’s usually some thinking or reason behind it. Possible a payment can be applied against multiple orders and that’s why it’s a transaction and not just a payment?

It’s an artifact of some of the old models we have hanging around in the system.

If you had, say, a $5 order and a $10 order, you could have a single $15 charge that applied to cover both. Although the models represent this, it always was an unusual use-case.