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 this file. This is a zip file that you may need to "unblock" before unzipping. Once downloaded, right-click the file and select Properties, then click the [Unblock] checkbox. Then uzip the file.
The extracted file is named "ProjectItems.chm" and it is a compiled HTML help file.