Some of these panes this package supplies itself, registered the first time their command runs. The rest a host supplies (Studio does): until one is registered under the key, the commands that open it are disabled.
Not every pane is here. A plugin registers its own panes under keys of its own, so a pane key is any string; these are the ones this package knows by name.
The keys of the task panes this package's commands open. Activate one with
taskPaneArea.activate(TaskPaneKey.Comments); register a pane under one to supply it or replace it.