update-fonts-scale assembles a fonts.scale file in an X font directory using one or more scale 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 scalable fonts (whose X LFD font names are not in the font files themselves) for the X Window System, but may be invoked at any time to reconstruct fonts.scale 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-scale__ will assemble /usr/lib/X11/fonts/directory/fonts.scale from the files found in /etc/X11/fonts/directory/package.scale''.
This enables multiple packages to provide names for fonts in the same directory. No font package actually provides the fonts.scale file in the X font directory itself, so there is no danger of overwriting one package's font names with those of another.
For instance, the two packages /usr/lib/X11/fonts/Type1. They each place their fonts.scale files in /etc/X11/fonts/Type1/xfonts-scalable.scale and /etc/X11/fonts/Type1/xfonts-nifty.scale (respectively). update-fonts-scale concatenates these two files (as well as any others that match /etc/X11/fonts/Type1/*.scale) into /usr/lib/X11/fonts/Type1/fonts.scale''.
The format for fonts.scale files is given in the mkfontdir(1)? manual page.