I know of no general list of the .INP content.
But by comparing the .INP content with the report display you will quickly see what is what.

The program formats the input echo based on the active .INP file in the \SYSTEM folder. Some reports offer several formats - for example, the ELEMENT list had ELEMENT.INP, ELEMENT0.INP, ELEMENT1.INP, etc. The program uses the file without the number in the name. If you wish to use a different "template" just copy one of those other ELEMENT files into ELEMENT.INP. (ELEMENT0.INP is a duplicate of the shipped ELEMENT.INP.) You can build your own templates.

Again, Stoomwezen has a special report to display its unique data.