What is a List Field?

With list fields, we mean fields that have multiple values. Usually fields such as emails, phone numbers and addresses are lists and each value has a label to differentiate between ‘home’, ‘work’ and ‘other’ for example.

When list fields are included in the default field mappings, we always sync the first value in the list, unless we sync between two list fields. In that case, we sync all values. 

You are not able to sync list fields in the customizable field mapping.