FortiManager supports network operations use cases for centralized management, best practices compliance, and workflow automation to provide better protection against breaches.
After importing policy for a newly added device and then attempting to install the new policy package to the FortiGate for the first time, all unused objects will be deleted.  
On the policy package import process we can select one of the following options:

1.- Import only policy-dependent Objects

When you select import only policy-dependent objects, the orphan (unused) objects that are not tied to policies locally on the FortiGate will be deleted on the next install. 

2.- Import all Objects

If you choose to import all objects, then all used and unused object in the FortiManager ADOM object database are imported, but it will still deleted orphan (unused) objects locally on the FortiGate on next install.

In this scenario, all unused objects are imported to the FortiManager database and latter they can be used by referencing into the FortiManager policies and install on the managed devices.