We are trying to specify the System.Iteration field to be READONLY until the workitem reaches a specific task. It seems this is a special field that doesn't allow it.

I can make the field readonly on the form but it cannot be enabled afterwards. Is there a way to accomplish this task?

Is there a documentation that provides the exceptions for special fields we have very little or no control at all?

Looking through the posts it seems people want to have most control over fields Team Systems folks deemed as special and as a result restricts the control over them. We are encouraged to reuse existing fields, we cannot control some of them, but there are restrictions on some of the field types one can have (i.e. no user-defined WorkItemClassificationControl).