Customer IDs inside ChargeDesk will match those on your payment gateway. When you create a new charge or new customer through your payment gateway, you'll receive an ID to uniquely identity the customer. You can use this ID with ChargeDesk to show things like the customer's billing history or latest invoice.
Typical customer IDs look something like this;
cus_4qUaA3zmeu3Cxa // Stripe customer ID AE9VWFN2RJNZK // PayPal customer ID 68343244 // Braintree customer ID
Some gateways, such as Braintree will allow you to generate your own customer IDs if you choose to do so. If your customer IDs are not random and can be guessed easily (such as an email address), we recommend activating high security when using ChargeDesk's self-support pages.