+.Ss \&nr
+Define a register.
+A register is an arbitrary string value that defines some sort of state,
+which influences parsing and/or formatting.
+Its syntax is as follows:
+.Pp
+.D1 Pf \. Sx \&nr Cm name value
+.Pp
+The
+.Cm value
+may, at the moment, only be an integer.
+The
+.Cm name
+is defined up to the next whitespace.
+The following register
+.Cm name
+requests are recognised:
+.Bl -tag -width Ds
+.It Cm nS
+If set to a positive integer value, certain
+.Xr mdoc 7
+macros will behave as if they were defined in the
+.Em SYNOPSIS
+section.
+Otherwise, this behaviour is unset (even if called within the
+.Em SYNOPSIS
+section itself).
+Note that invoking a new
+.Xr mdoc 7
+section will unset this value.
+.El