WHMCS by default only allows one callback URL to be entered in the Callback Settings under the master settings. This URL is typically used by payment gateways (e.g., PayPal, Stripe) or provisioning systems to notify WHMCS about events like payments, cancellations, or server creation results.
If you're running multiple WHMCS installations (or VPS provisioning endpoints), and you'd like to route callbacks dynamically depending on which WHMCS instance or VPS provider was involved, you'll need to use a workaround or implement custom logic.