Build times are reduced with extensions: Build and compile times are reduced with extensions because you don’t have to compile the model that is being extended.
Enums, where the IsExtensible property is set to True, can be extended: The IsExtensible property of the enum must be set to True to be able to create an extension.
Check that you’re referencing the model that houses the element that you want to extend: To create an extension of an element, you must verify that you’re referencing the model in which the element resides.