Relation between models in Ax 7

I recently upgraded my 2012R3 code to Ax 7.  I've fixed conflicts outlined in "CodeMergeSolution" solution.

I can see that my customizations are spread out in different model : ApplicationPlatform.MyModel, ApplicationFoundation.MyModel, ApplicationSuite.MyModel, ...

SysUserInfo contains a custom field, so it appears in ApplicationPlatform.MyModel since SysUserInfo belongs to ApplicationPlatform.  But my custom field uses an EDT that exists in ApplicationSuite.MyModel so it doesn't see that EDT!  

Questions :

• Should I move my EDT to the ApplicationPlatform.MyModel?
• If I do, will my EDT be available to all other Model (since it's the lowest model) or I will cause the same problem in ApplicationSuite.MyModel because the EDT is used there too?

I'm still trying to grasp all this new model structure.  Thanks for your help.