Custom Apex REST is a way of creating custom RESTful web services using Apex code. Custom Apex REST can be used to expose Salesforce data and functionality to external systems, such as the product information management (PIM) system. By using custom Apex REST, the developer can create a custom endpoint that can accept requests from the PIM system, and perform the required operations on the Product2 and PricebookEntry objects in Salesforce. Custom Apex REST can also use the custom setting to determine the Pricebook2 to use for creating or updating the PricebookEntry records. Custom Apex REST can also handle authentication, authorization, and error handling for the web service. Reference: [Custom Apex REST Web Services], [Create a Custom REST API Endpoint]