Semantic Objects and Action for and an app in SAP FIORI Launchpad

In SAP Fiori Launchpad configuration we use semantic object and Action. I am trying to understand that what a developer should do if there is need to create one for either t-code or WebDynpro app.

1. What will happen, if we assign Semantic Object and Action for more than one FIORI app?
2. In an organization, who is responsible for creating of these object (I am trying to understand the process, how a developer tackle a situation where there is need to create these objects for an app)

Thanks,