Am I overlooking something obvious, or is this undocumented?

Pulse Energy

We may have missed documenting this one. The rule is used for these CSS properties:

@prince-pdf {
prince-pdf-open-action: ...
prince-pdf-page-layout: ...
prince-pdf-page-mode: ...
prince-pdf-paper-tray: ...
prince-pdf-print-scaling: ...
prince-pdf-script: ...
}