Based on what my defect count is looking like, a transaction is considered an item, whether the buyer paid for them all together or separately.

I have one buyer who left a low DSR for description on multiple items that they purchased from me, all of which were paid for with a single payment. While my defect count in total was from less than 5 buyers, it was being counted as one transaction. Then I had a buyer leave a neutral which meant all of my defects came from 5 different buyers (that's when it can affect whether you're TRS or not).

As soon as that happened, it stopped showing my defect rate as from 4 transactions and counted every single item from that order. For example, say they bought 10 items, the other buyers only bought one each. Before the fifth, it said "four of your transactions had defects", after the fifth, it said "transaction defect rate 14 / xxxxx transactions had defects.