Apparently there are two standards for control sequences generated by the numeric and editing (INSERT, HOME PAGEUP, etc) keypads. First is the standard mimicking VT220 and tested with vttest. Second is the standard to which XTerm adheres by default called PC/Sun. The differences are described in XTerm spec. All this warrants "Keyboard compatibility" configuration option in the SETUP.

When testing PC/Sun standard I also discovered that the spec has an error in defining application controls for the numeric keypad! END and HOME keys correctly specify SS3 prefix and the renaming cursor keys incorrectly specify the CSI. The actual behaviour of XTerm on my computer confirms this.