NAV Synchronization

Hi,

I am using NAV 2009 SP1 with SQL Server 2008. I would like to ask a few simple questions with respect to Synchronization.

When is Synchronization exactly required in the NAV db for the following scenarios:

1. When new user is created in the NAV db and roles assigned to the user
2. When the permission in a specific role has been changed for an existing user
3. When the roles of an existing user is being changed in the NAV db

Secondly what would be the impact of these cases when the security model is set as Standard or Enhanced

Thanks in advance!