The "Placeholder Images" section of the PureClarity configuration page in Magento 2 is where you can set fallback image URLs to be used when images are not present.
Product image placeholder
This image URL will be sent to PureClarity if no product image is set. If no value is set here, the Magento default placeholder will be used.
Category image placeholder
This image URL will be sent to PureClarity if no category image is set. If no value is set here, the Magento default placeholder will be used.
Secondary category image placeholder
This image URL will be sent to PureClarity if no category "Override Image" is set (see "Category Attributes"). If no value is set here, no value will be sent to PureClarity.