What does 'Product not available in Category (or) Collection' mean?

When particular products that are pinned, are removed from the Category or Collection (changes in the condition or removed manually), the message 'Product not available in Category (or) Collection' is displayed.  These products will not be available for merchandising in the Category or Collection. 

The below example is from a Magento demo store. 

  • The pinned products will be visible on the 'Curate' panel with the message 'Product not available in Category'. 

  • This could be either because the products were removed from the Category or the conditions using which the products were chosen might have been changed.