Data models are collections of tables with information about the same purpose of analysis (e.g. information about a Purchase - To - Pay (P2P) processe structured within several tables).
The tables available for creating a new data model depend on the data store that is chosen as data source during the creation of the data model.
Data Models & Analysis Documents
To understand the relation between data models and analysis documents, you should consult the Structure chapter.
In the project navigation you can find two sections, that are assigned for analysis documents and data models.
As you can see on the screenshots, this project contains several analyses, but only one data model, which means, that all analysis documents rely on this data model.