For details on how product recommendations work in Clientbook please view the article titled "How Product Recommendations Work"!
In Lightspeed R-Series, there's a product field called "Manufact.SKU", which is used to generate your product recommendations in Clientbook.
To ensure accurate recommendations, make sure this field only includes the exact vendor item ID (also known as the Manufacturer SKU or Manufacturer Part Number/MPN).
For more details on how to add a new product in Lightspeed R-Series, check out the article "Adding items manually"