"Gold Modelstore"

During your R3 Upgrade Project, you will likely need to update code.  This might be hot fixes, updating customizations to reflect changes due to R3, doing necessary code merges due to ISV or VAR code...

Export the modelstore and use to update the R3 Test environment, and eventually use for the Production environment upgrade in-place.

 Excerpt from https://technet.microsoft.com/en-us/library/jj733502.aspx

Creating the R3 Test Environment and Gold modelstore vs. Upgrading Prod

As I've stated in previous post, upgrading to R3 is an arduous process.

The steps that I stated earlier in a prior blog post  was to create the initial R3 Test environment and to create the "Gold" modelstore.  After that is completed you can clone it and create a Development environment.

Once that is completed, the project team can begin to regression test R3 with company data.  Customizations can be validated and modified if necessary.  Code updates can be applied and modelstore updated.

For the production upgrade, you will follow similar steps as previously noted but instead of installing vanilla AX, applying models, etc, your DBA will "upgrade" the modelstore using the modelstore from your Dev/Test environment.

From Technet article on Performing in-place upgrade to AX 2012 R2/R3