Blame man(7), have a look at mdoc(7):
Semantic markup for command line utilities:
Nm : start a SYNOPSIS block with the name of a utility;
Fl : command line options (flags) (>=0 arguments);
Cm : command modifier (>0 arguments);
Ar : command arguments (>=0 arguments);
Op, Oo, Oc : optional syntax elements (enclosure);
Ic : internal or interactive command (>0 arguments);
Ev : environmental variable (>0 arguments);
Pa : file system path (>=0 arguments)
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