What type of validation can be defined in addition to built-in validations?

Custom server-side validation is an essential output for ensuring that the data being processed adheres to specific business rules and constraints that may not be covered by built-in validations. This type of validation occurs on the server, which adds a layer of security and correctness to the data before it is stored or processed further.

By implementing custom server-side validation, developers can create rules tailored to their application's unique requirements. For instance, this could involve checking if a certain field meets a specific format, ensuring no duplicate records are submitted, or verifying that the input data corresponds to business logic, such as confirming a user's eligibility based on multiple fields.

While built-in validations cover many common cases, they may not address complex validation scenarios specific to a given application. By allowing custom server-side validation, OutSystems encourages developers to enhance their applications' integrity and robustness.

In contrast, while client-side validation and custom client-side validation certainly have their places in user experience by providing immediate feedback to users, they can't replace the necessity of server-side checks. Relying solely on client-side validation can lead to security vulnerabilities because it can be bypassed by users. As for the option suggesting no additional validations are allowed, that contradicts the flexibility provided within the OutSystems framework for building robust applications