It exposes core business logic and data allowing the integration with other systems, mainly mobile and frontend applications, using REST web services. 

The commercefacades extension exposes a group of facades which simplify the communication between the store frontend and the service layer. This is a summary which I wrote while studying for the certification. The Hybris Documentation about commercefacades has one page and it is worth reading.

This extension enables the management of customer sales promotions.

What contains each field of the order and its entries? I answer this question often, so here is the explanation.

The main goal of the Hybris eCommerce Suite is to integrate the order process of a company. Here is the list of the features offer by this module to towards this goal. In every case, further customization is required.