When you create a segment and add conditions for the Placed Order or Ordered Product metrics, you can add filters to get even more granular.
One option is to limit your segment to only capture those who placed an order and purchased a specific item(s). If you add such a filter and click to enter a dimension value, you will see a drop-down menu populate with all available values (i.e. all item values in Klaviyo).
Klaviyo will automatically populate this drop-down with all products and product categories that have been purchased or purchased from. This means if you have new products that have not yet been purchased, they will not appear in this drop-down.
Given the above, if you don't see a product or category listed, this usually means it hasn't been purchased yet.
You can still use these values to filter a segment (or flow). Simply copy and paste the product name in the value field and save it. So long as the product name you copy is identical to the product name in your store, the segment will recognize the value and map it to the right product as soon as it is purchased for the first time.