ARGUMENTS(MDOC) MDOC ARGUMENTS(MDOC)

style/arguments
command line arguments

.Ar placeholder 
.Ar 
.Cm fixed string

If a utility takes an arbitrary number of arguments, all of which are interpreted as file names, use the Ar macro without arguments, for example:
.Sh SYNOPSIS 
.Nm rm 
.Op Fl dfiPRr 
.Ar 
.Sh DESCRIPTION 
 
.Sh SYNOPSIS 
.Nm cat 
.Op Fl benstuv 
.Op Ar 
.Sh DESCRIPTION
Without arguments, the Ar macro formats in the same way as
.Ar file ...
If an argument is a fixed string or a keyword and is not to be replaced with a value chosen by the user, the Ar macro is inadequate. Use Cm instead.

The MACRO REFERENCE section in the mdoc(7) manual.
September 3, 2015 bsd.lv