When moving over to Bookem, you may have some patients with credits where they have paid in advance for appointments or returned items. Naturally you would like to reflect these balances on Bookem, but do not want them to reflect as payments as they would then be in your accounting system as duplicates and it would appear you earned the money twice.
To carry these over smoothly, create a payment method called "opening balance" then record credit transferred from your legacy system for each client using the "opening balance payment method"