This tickbox field on the Contacts module works in conjunction with the sync options available with our integration with Microsoft 365, Googlesync, and the Outlook Add-In
If a record does not have the "Sync to Outlook" box ticked, this will contact will automatically be excluded when using the Outlook Add-In.
Googlesync / Microsoft 365
On each User settings, you have the option to ignore/ override this tickbox
More information about the Outlook Add-In: can be found here.
See Also: this FAQ
More information regarding Googlesync: is also available.