AX 2012 Model store database and Baseline model store Database.

Model store database
The model store is the database where all application elements for Microsoft Dynamics AX are stored. Customizations are also stored in the model store. The model store replaces the Application Object Data (AOD) files that were used in earlier versions of Microsoft Dynamics AX.

Baseline model store
The baseline model store holds model store tables for the earlier version of the metadata and is used only during upgrade. The baseline model store is like the old folder in earlier versions of Microsoft Dynamics AX.

Note : In Microsoft Dynamics AX 2012 R2, the model store was moved into a database that is separate from the business database.