The Data Explorer contains all of the available fields for use on custom reports.
For a list and descriptions of these fields, download and extract ReportutilitiesandProjectItems.zip. You may need to "unblock" this ZIP file before unzipping.
Once downloaded, right-click the file, select Properties, and click the Unblock checkbox. Then, unzip the file.
The extracted file is named ProjectItems.chm, a compiled HTML help file.