I believe the most commonly used target atoms are STRING, UTF8_STRING, and any MIME types such as text/html or text/uri-list, but I think technically it can be any string. As long as the drag source and destination can agree on one of the provided target types then the drag can happen.
If you’re writing an app that accepts drag data from external apps, you’ll have to find out what data types those external apps offer. It’s been a while since I’ve done this but check out the low-level drag-motion signal to get the GdkDragContext, then call gdk_drag_context_list_targets.
I’m not too sure about info. Most code I’ve seen start at 0 and go up but it doesn’t seem to really matter.