The "Mode" section of the PureClarity configuration page in Magento 2 deals with the mode that PureClarity uses to get recommenders.
There are two modes:
This is the default mode for PureClarity.
In "Client-side" mode, all data and HTML for recommenders comes from the PureClarity servers when on the frontend of the site. No extra data for elements displayed in the recommenders is pulled from Magento at the time of display.
All recommender template HTML is managed via PureClarity admin.
In "Serverside" mode requests for recommenders are routed through Magento to ensure that recommenders are populated using Magento data after getting SKUs to show from PureClarity. This is ideal for when you have complex and/or custom pricing rules & logic or non-standard restrictions on what products customers can see.
For more detail on how to customise the look of product recommenders, and how to override or add your own data see the serverside mode page.