In RTC, what types of plan attributes are editable withing the plan columns?

Kim,

as far as I can tell, which attribute types are editable it is not necessarily related to the display type. It depends if there is a special editor presentation for plan views for the attribute type that allows editing and not only display. I am not aware of a complete list of types, but my experience is that the following are editable

1. Some built in attributes such as summary and enumeration based built in attributes.
   
2. Enumeration type attributes seem to be editable in general if I remember correctly

1. List types, also enumeration list
   
2. Text and HTML types except the ones with special support
   

Keep in mind that the plan view is supposed to provide you with some compressed view of the most important attributes of work items and is not designed to provide you with the greatest edit experience and there actually is an in plan editor that you can use that supports more attribute types as far as I can tell.

You can only add supported attributes/attribute types to the view. Some will not show up and are apparently not supported and you can not change that either.

It is not possible to make attributes editable using some customization the attribute is either editable or it is not and you can't change that (you can create an enhancement request).