The situation is quite complicated to me, let's say:

1. I have 3 product categories.
2. I only want to track purchases.

However, if you insert the tracking snippet into the thank you page, each product category will trigger the same tag. customers who buy dresses and customers who buy guns may look totally different. so, if you use the same thank you page to track the conversions, the Google Ads AI will have trouble in optimizing your campaigns:

• A. Dresses sales campaign
• B. Guns sales campaign

When people buy a dress, the dress conversion and gun conversion will both increase by one, although the transaction amounts are different by default. This shows that the conversion is contaminating with another campaign (the gun sales campaign).