With field synchronization in CRM 2015, admins can set the sync direction between Microsoft Dynamics CRM and Microsoft Dynamics CRM for Outlook fields. You can control synchronization when using either Outlook synchronization or server-side synchronization (Exchange).

For example, a salesperson may want to take personal notes about a contact and not want the notes to synchronize with CRM data available to all users. You can set the Personal Notes field for contacts in Outlook to not CRM for Outlook with CRM so the salesperson’s notes will remain private.

In CRM, choose Microsoft Dynamics CRM > Settings > Email Configuration > Email Configuration Settings

1. Choose the Synchronization tab > synchronized fields

2. For the fields you want to change synchronization, choose the arrows in the Sync Direction column. Each choice will change the direction.

   

   Hover over a field name to see the fields mapped to it.

3. Choose OK > OK to close the open dialog boxes.

Permissions and synchronization

Role-based security controls access to a specific entity type, record-based security controls access to individual records, and field-level security controls access to specific fields. All these can impact what is synchronized between CRM and CRM for Outlook or Exchange.

Source: Technet

Hope it helps!