Product Content Types are actually just DocTypes. But these DocTypes are specifically used to display custom product or product variant properties that can be edited on either your product or product variant. They work in a similar way to Nested Content, and allow Merchello to render the Umbraco property editors on a product or option.
An example would be, that you want to add an SEO tab and an Images tab on your product. You would create a DocType with these properties and tabs and name it whatever you want (i.e. Product - Standard).
To tell Merchello you want to use this DocType, you need to register it as a Product Content Type. Simply right click on the 'Products' root and choose 'Product Content Type'
Give it a Name and choose the DocType from the dropdown at the bottom, then click Save. This is now available to us to add to a product.
To add this to a product, just go to a product and edit it. You will see a Tab called 'Extended Data' from here all you need to do is select the DocType you just created (Product - Standard) and click the icon next to it.
Now you'll see your DocType properties rendered and you can update and save them just like a normal Umbraco node.
This same logic applies for product options, and you can have custom properties on different options.
Updated less than a minute ago