Where can I find an item's product posting groups?
You want to check the posting-group fields included in the POS365 item setup.
Note: This FAQ uses the POS365 Manager role and English page names. If your screen looks different, follow Access the POS365 UI (user interface) Manager role in Business Central. Changing the language to English is optional, but makes it easier to follow this FAQ.
Answer:
In POS365 Navigation, under Products, select Items and open the relevant item. Scroll to Costs & Posting and expand it. Select Show more in that section if needed. Under Posting Details, check Gen. Prod. Posting Group and VAT Prod. Posting Group.