The columns you'll see in a CSV file when importing/exporting Customers are as follows:

• id: The unique ID Syncro assigned to that customer. We strongly recommend you do NOT modify this.
• firstname: The customer's first name.
• lastname: The customer's last name.
• email: The customer's email address.
• address: The main address for the customer.
• address_2: Used for apartment number, etc.
• city, state, and zip: The customer's location information.
• phone: A phone number serves as a unique ID for the customer. We recommend you include one.
• mobile_phone: A mobile phone number is required for SMS.
• office_phone: Phone number in the Office Phone category.
• home_phone: Phone number in the Home Phone category.
• fax_phone: Phone number in the Fax Phone category.
• other_phone: Phone number in Other Phone category.
• get_sms: Set to True if the customer can receive SMS messages.
• location_id: We recommend you do NOT modify this.
• business_name: If the customer has a business, or if this is a business account, include the business name here.
• created_at: System driven. We recommend you do NOT edit this unless you specifically need to.
• updated_at: System driven. We recommend you do NOT edit this unless you specifically need to.
• referred_by: You can customize referred by sources in Admin > Customers - Preferences > Additional Referred By items. These sources show up in the Referrals report (Admin > Reports > Customers - Referrals).
• notes: Notes on Customer detail.
• tax_rate_id: Tax rate for the customer if different than the default. The tax rate is the code in the URL when viewing Tax Rates.
• tax_free: Set to True if the customer is tax-exempt.
• no_email: Set to True if the customer should NOT receive email or False if the customer wants to receive email.
• opt_out: Set to True if the customer wants to opt-out of marketing emails or False if they want to receive them.
• disabled: Set to True if the customer should be disabled. (This is the equivalent of deleted.)
• custom_fields: These use the following format, where each field:value is separated by a semicolon:
    field name:value;next field:value.

Notes:

• If you delete the id (which we do NOT recommend), make sure the Name, Phone and Email are unique enough so as not to overlap entries.
• Some Syncro users have found it necessary to supply opt_out values (true or false) to import successfully. If you have issues importing, add data to that column for each customer based on their preferences.