pxboard is a shell script that, according to the comments in it, can be used to let XBoard display a game through a command
cat GAMEFILE | pxboard
I thought this was totally obsolete. For one, the smae can be achieved by simply typing the shorter
But perhaps the intended use is when the game isn't supplied by a simple cat command on a file, but by some other method. Like output of a mailing program.
But nowadays almost no one would use mailing programs from the command line. If they get sent games by e-mail (does anyone still do that???) he would probably copy-paste those from his e-mail client (or from a webmail or other www page) into an xboard instance he started separately. If games are in a PGN file, he could double-click that file to launch XBoard with that file as argument, as the PGN mime type gets associated with XBoard on install.
I don't know what was the original idea for installing pxboard. Should it be moved to $bindir and be given executable mode, so that 'pxboard' would work as a command on the command-line? If so, "make install" should do that, but no longer does.
Conclusion: either the Makefile should be adapted to do that, or all references to pxboard should be removed from the man pages.
Ubuntu probably chose not to package pxboard. You can get it here:
The man page for xboard describes piping with the pxboard script.
pxboard is not on my system. I am running Ubuntu 18.04.