When you create a user-defined property type there are two things to keep in mind.
- The user-defined property should be transient.
- The user-defined property must have a data-type defined or the item that contains this property will not be displayable in the ACC.
Here is an example of a user-defined property type.
To learn more, User-Defined Property Types. Note that in the ATG documentation the example for the user-defined property does not define a data-type. This is probably a documentation bug.