- directories, 0644 for all other file
- objects. For z, Z lines, if omitted or when set
- to -, the file access mode will not be
- modified. This parameter is ignored for x, r,
- R, L lines.</para>
+ directories, 0644 for all other file objects.
+ For <varname>z</varname>, <varname>Z</varname>
+ lines, if omitted or when set to
+ <literal>-</literal>, the file access mode
+ will not be modified. This parameter is
+ ignored for <varname>x</varname>,
+ <varname>r</varname>, <varname>R</varname>,
+ <varname>L</varname> lines.</para>
+
+ <para>Optionally, if prefixed with
+ <literal>~</literal>, the access mode is masked
+ based on the already set access bits for
+ existing file or directories: if the existing
+ file has all executable bits unset, all
+ executable bits are removed from the new
+ access mode, too. Similarly, if all read bits
+ are removed from the old access mode, they will
+ be removed from the new access mode too, and
+ if all write bits are removed, they will be
+ removed from the new access mode too. In
+ addition, the sticky/SUID/SGID bit is removed unless
+ applied to a directory. This
+ functionality is particularly useful in
+ conjunction with <varname>Z</varname>.</para>