See https://developers.google.com/drive/manage-comments for details.
FWIW, anchors are a bit limited at the moment. The biggest issue is anchors on comments are immutable. If you end up using a custom scheme and can refer to something uniquely identifiable in your content (e.g. an XML element ID or some other marker) then you shouldn't have any issues. But some of the other anchor schemes detailed on the doc are problematic. For example anchoring to a line in a text file will break if the file is modified an a line is inserted before the anchor location. Until anchors are mutable, best to limit your self to custom schemes and/or unanchored comments.