What is .pline ?

At https://www.gnu.org/software/groff/manual/groff.html (5.37 GNU troff 
Internals), there is an example using the ".pline" request; this is 
supposed to display internal nodes:

    printf 'Gi\\[:u]\\%%seppe\n.pline\n' | groff -z 2>&1 | jq

With groff 1.22.4 this fails because groff says .pline is an unknown 
command.

I thought maybe a.pline request which outputs was added in a more recent 
release, however "pline" is not described anywhere in the above html manual.

(Also: Was -z or -Z intended?  "jq" is a JSON processor on my system.)