What are sync settings?
FullContact offers two settings for your connected address books (Google Contacts, iCloud, and Exchange): ON or OFF (see image above).
This can be set for each individual address book. If toggled on, it means contacts will be added to your My Contacts list, as well as combined with any other connected address books also set to on. These contacts will sync back to your connected sources. In other words, for as many address books that you've connected (and turned syncing on for), expect that they will match/mirror each other.
If toggled off, you are essentially making that connected source a part of your FullContact account, but those contacts will only be used to update existing contacts in your My Contacts. The changes you make to these contacts will sync bi-directionally, but not with your My Contacts. In other words, they are kept separate from My Contacts unless you manually add these contacts to your My Contacts.
Lastly, if you choose Update Existing instead of Add as New when importing your LinkedIn address book or a CSV/vCard file, we will only update your existing My Contacts with contacts in your imported file. However, if you choose to Add as New, expect that all contacts in your imported file will get added to your My Contacts. If there's overlap, FullContact will de-dupe your My Contacts.
How do I know which sync settings are right for me?
If you're looking to combine all of your address books (work/personal email sources), turn syncing on on your Settings page. This will ensure that each address book has the same contacts. So, if you make a change/edit to a contact in one address book (let's say it's your personal Google Contacts account), that change will also sync to your other connected/synced sources (let's say that's your work Exchange address book).
For LinkedIn (and any other file you import), it depends on whether you want ALL of the contacts in your LinkedIn address book/imported file to be added to your My Contacts.
For example, if you have your personal Google Contacts account connected/synced to FullContact and you don't want ALL of your LinkedIn contacts to be added to your My Contacts/Google Contacts, choose Update Existing. However, if you want all of your LinkedIn contacts added to your My Contacts, choose Add as New.
Remember, even if your choose Update Existing, you can always manually add contacts to your My Contacts.
Here's a helpful sync settings video!