Its internal info from FGW▸ , train diagrams with certain services and the train type basically. The last diagrams I saw had "Confidential" at the top, so you'd need to know somebody "inside" to access them.
Would a Freedom of Information Request supply this info? I recently requested from
TfL» the working timetables for London Underground and was supplied despite 'Confidential - Staff Use Only' printed on each one.
Slightly (but only just) more on-topic, have there been any cases where a train has been 'formed of 3 coaches instead of 2' (or a similar enlargened train instead of a shortened train)?
Point One: Unlikely, don't see why you need to know what a particular set is 'meant' to be working, baring in mind it doesn't always stick to the diagram.
Point Two: Yeah sometimes get extra carriages, I personally not seen it often though.