'salt' attribute affects a cache filename to generate different name from directory name.
This is useful when sharing caches with host on sandbox and/or give a filename differently:

    <dir salt="randomdata">/usr/share/fonts</dir>
    <remap-dir as-path="/usr/share/fonts" salt="salt for /usr/share/fonts on host">/run/host/fonts</remap-dir>

Applications can read caches as-is for fonts on /run/host/fonts where is mounted from host.
and write a cache for their own fonts on /usr/share/fonts with different name.