-file. The argument @var{user} can be either an existing user symbolic
-name, or a decimal numeric user @acronym{ID}.
+file.
+
+If @var{user} contains a colon, it is taken to be of the form
+@var{name}:@var{id} where a nonempty @var{name} specifies the user
+name and a nonempty @var{id} specifies the decimal numeric user
+@acronym{ID}. If @var{user} does not contain a colon, it is taken to
+be a user number if it is one or more decimal digits; otherwise it is
+taken to be a user name.
+
+If a name is given but no number, the number is inferred from the
+current host's user database if possible, and the file's user number
+is used otherwise. If a number is given but no name, the name is
+inferred from the number if possible, and an empty name is used
+otherwise. If both name and number are given, the user database is
+not consulted, and the name and number need not be valid on the
+current host.