Welcome to the wonderful world of the Apache OFBiz Data Model.
Much of this data model is based on "The Data Model Resource Book, Revised Edition, Volume 1" by Len Silverston ISBN 9781118082324. Volumes 2 and 3 of this series are also excellent resources, but volume 1 is the most important.
To help your understanding of the data model there are various documents and resources in the pages under this page.