Under Settings > Xero, click the Disconnect button next to the Organisation field:  

No. At this stage, we strongly recommend making all of your changes in Syngency, which will then be synced across to Xero. If you alter a contact or invoice in Xero, it won't be updated in Syngency. The exception is when you add a payment in Xero – this information is sent back to Syngency to update bookings, reports, and statements.

Typically within 15-20 minutes of entering the payment. We are working on reducing this to just a few seconds in future.

These fields allow you to use special codes to specify what information is shown in the Reference field for Invoices and Bills, and in the individual line items for each.
For instance, by default Syngency uses the talent's name and the fee description as the line item description in Xero:

Sia Anderson - Half day shoot

Using special codes in the Line Item Description field, you could alternatively add the shoot date, job number, and first three letters of the clients' name instead, so it looks like this:

Sia Anderson - CLE 11/12/2018 (#AH1234)  

Contact our support team via Intercom if you would like help in setting this up.

Billing Codes let you create various options that can override the chart of accounts settings you define when you first connect Syngency to Xero.

For instance, you may have account 261 in Xero assigned to "Agency Expense (Model)" (expenses charged to the model by the agency):

But in the case of it being a cash advance to the model, rather than an expense, you may want to use an alternative Xero account when entering this in Syngency.
First, you would add it under Settings:

And then each time you enter a Fee, Expense, or Chargeback, you have the option of selecting a Billing Code to assign it to the respective Xero account:

Syngency syncs with Xero whenever any of the following are saved:

The options on the Settings > Xero page in Syngency dictate which accounts in Xero Syngency codes transactions to when the data sync takes place.

Syngency polls Xero for new payment information every 15 minutes.

Any staff added to a Billing Department will be synced to Xero as Contact Persons when the contact is saved in Syngency.