What is one of the limitations when modifying a CDT in Appian?

The correct answer highlights that updates to a Complex Data Type (CDT) in Appian must be made by editing the XML schema document. This process is necessary because CDTs are defined using XML schemas, and any changes to the structure, such as adding or removing fields or changing data types, must be reflected in the underlying XML.

When you modify the XML schema document, you are directly controlling the definition of the CDT, which allows the changes to be accurately incorporated into the Appian environment. Once the XML is updated, the CDT can then be re-imported and published within Appian to reflect the new structure. Making changes through the XML file ensures that the integrity and relationships of the data types are maintained.

The other limitations related to CDTs revolve around their management and versioning. It's also important to note that while CDTs can be modified, it entails technical steps including XML editing, and this requirement might not be part of every developer's workflow. This highlights the technical nature of working with CDTs in Appian.