Possible outputs are:

1) Display (implemented)
2) WYSIWYG (needs to be merged with existing mechanism)
3) Preview (implemented)
4) PDF export
5) HTML export
6) Email
7) RSS
8) No display

"No display" will be used when the renderer is drawing a page post-edit to extract links, and run the macros in case they need to update metadata - so display macros can just shortcut their output, especially macros that need to make external connections.