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In the context of Client Scripts in ServiceNow, the onSubmit type of Client Script is designed to manage actions just before a record is submitted to the server. It is commonly used for validating data, confirming user input, or performing checks that must occur immediately before data is saved. Callback functions, which allow for asynchronous operations or deferred execution after certain actions (like API calls), are typically not appropriate in this context because the onSubmit function is expected to complete its operation synchronously.

Using a Callback in an onSubmit script could result in inconsistent behavior or data being submitted before the callback has completed, thereby failing to enforce necessary checks or validations. The primary goal in onSubmit scripts is to ensure that validations or other synchronous logic have completed before allowing the record to be processed, emphasizing the need for immediate execution of the defined logic.

Other types of Client Scripts, such as onLoad, onChange, and onCellEdit, allow for user interface interactions and can benefit from asynchronous operations, making callback functions more suitable in those contexts. This aspect of the onSubmit type highlights why a Callback function should not be utilized there.