I am not sure if I got understood your question. But in case, you are looking for the details of a field (of an extended schema), you can check the 'Preview' tab of the main schema. (e.g. if nms:recipient schema has been extended to cus:recipient, the extended field details can be viewed in Preview tab of nms:recipient schema).
In addition to this, if I create a variable within a workflow, let say myCode in an activity,
and then later if I want to use this code, then i have to use it as targetData.myCode, am I right?
Then if I need to create a query then I have to use [targetData/@myCode], is this ok?