Welcome to the forum!
That is for custom markers, and the label does appear for markers you make yourself. The only thing I could suggest is to make yourself a resource kind of document, where you use all of the built in markers on the canvas. Then once they are on the canvas, you can look in the XML Editor to find the name of each one.
I did find a file markers.svg in the install directory Inkscape/share/markers. But when I open it, the canvas is blank. When I open the XML Editor, it is showing all the markers there, but I don't understand why you can't see them on the canvas. It must be some programmer magic. (I saw something similar when I installed a custom symbol file, but I don't understand how it works.)
I also found, in Inkscape/docs a file called markers_design.txt. It give some kind of general info for designing markers, and might help you indirectly.
I guess your client must have some SVG files, and is looking into the XML to find the name of a marker. If they could share that file with you, that might help as well.
Oh, I just found another file that might help you. It's also called markers.svg but it's in Inkscape/share/examples. It looks like just a bunch of markers on the canvas, and if the one you need happens to be there, you can use the XML Editor to find out.
Sorry, I wish I could be more helpful.
