Plan custom attribute
Accepted answer
There is no public planning API and there is no way to create a custom attribute that is associated to a plan (as opposed to a work item attribute to be shown in a plan).
There is also no API that would allow to actually do anything with such an attribute.
One other answer
You can add the custom attribute in the plan's attribute mapping.
Please see the answer from https://jazz.net/forum/questions/103578/rtc-how-to-show-custom-fields-as-columns-in-plans