$$content$$ to access the content schema data of a metadata item, means the virtual item content or e.g. Exif, IPTC, DublinCore information for files.
$$metadata$$ access to the layer schema data of a Content Item, so all metadata which is added as Layers.
$$audit$$ to access audit information like created by user (including user details name, first name, email), modified by user (including user details name, first name, email), modification date or creation date.