Facebook’s pixel has an attribute named content_ids (required for Dynamic Ads), which requires an Array of content IDs as its value. It’s very possible you’re running this pixel on a site which already has Enhanced Ecommerce for Universal Analytics implemented, and now you want to use the same Enhanced Ecommerce data that your developers have already made available in this Facebook pixel.
Or perhaps you want to concatenate a list of strings, such as article tags (['culture', 'politics']), and send it as a comma-separated string to Google Analytics ('culture,politics').
