update-fonts-alias assembles a fonts.alias file in an X font directory using one or more alias files found in a subdirectory of /etc/X11/fonts/. It is typically invoked only from the post-installation and post-removal scripts of a package containing fonts (that provide aliases for their names) for the X Window System, but may be invoked at any time to reconstruct fonts.alias files. For each directory, which is either an absolute path to an X font directory or (preferably) simply the last component of its path (such as update-fonts-alias__ will assemble /usr/lib/X11/fonts/directory/fonts.alias from the files found in /etc/X11/fonts/directory/package.alias''.
This enables multiple packages to provide aliases for fonts in the same directory. No font package actually provides the fonts.alias file in the X font directory itself, so there is no danger of overwriting one package's aliases with those of another.
For instance, the two packages /usr/lib/X11/fonts/misc. They each place their fonts.alias files in /etc/X11/fonts/misc/xfonts-base.alias and /etc/X11/fonts/misc/xfonts-cjk.alias (respectively). update-fonts-alias concatenates these two files (as well as any others that match /etc/X11/fonts/misc/*.alias) into /usr/lib/X11/fonts/misc/fonts.alias''.
The format for fonts.alias files is given in the mkfontdir(1)? manual page.