To automatically execute an Azure Function when a customized item requires processing, the function must be configured with a Trigger. In this standard event-driven architecture, item customizations are queued as messages, making the Service Bus the event source that initiates the function's execution. While the function will ultimately save the processed data to Azure Cosmos DB via an output binding, the mechanism required to initiate the processing of the incoming item is the Service Bus trigger.

Microsoft Learn: Azure Functions triggers and bindings concepts (Section: Triggers and bindings overview). Defines that a trigger dictates how a function is invoked and that every function must have exactly one trigger.

Microsoft Learn: Azure Service Bus bindings for Azure Functions (Section: Service Bus trigger). Details how Service Bus triggers allow Azure Functions to automatically react to and process new messages arriving in a queue or topic.