That depends on what type of class it is. That specific class looks like a form DTO, so I would probably put that in the 'site' project. In general, classes should go like this:
admin - all admin-specific code (you probably won't have much, if any, of this)
core - all Hibernate entities, services, daos that both the admin and site application might need to use
site - anything related to Spring MVC in operating your frontend web application (@Controller, Form DTOs, validators)
Adding a task for us to add this in a bit more detail in the docs. Let me know if you have additional questions.
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