Information register purpose
Let us look at the Service document. As you remember, in this document you can select a rendered service and enter its price.
Obviously, the Jack of All Trades company has a price list for its services. It would seem that the price of services is an integral part of the services themselves and should be included as an attribute of the MaterialsAndServices catalog.
However, service prices can change over time. And you might need to make changes or corrections to previously posted Services documents. In this scenario you will be unable to get the correct price for the service because the catalog attribute stores the latest value.
Moreover, the Jack of All Trades management might request a report showing the dependency of the company profit on the service prices. So the ability to analyze the price changes over time is essential.
Therefore, to store the service prices, you will use a new object: Information register.